Don't show tracing tools by default when opening stack.

When opening a second stack, the tracing toolbox (toolbox_data) will
show, although it shouldn't.

project.js:setEditable() will unhide the tracing toolbox whenever a
stack is opened. setTool() will then hide the toolbox again, but only
if the stack opened is the first one (see project.js:70).

I didn't look into this any further, @tomka could you please check
if this is a valid fix?
Add missing closing DIV tag to statistics widget template
Statistics widget: remove superfluous width statement

This also makes the browser respect the margin statement which makes the
table look nicer by not reaching the right border of the widget.
Review system: add multi-user review model

This commit adds the database migration to add a separate review table to the
database. Each entry represents a review by a particular user for a particular
treenode of a skeleton. This new table has the following fields: reviewer,
reviewer_time, treenode and skeleton. Technically, the last field (skeleton)
isn't needed, because one can get the skeleton through the treenode. However, it
makes sense to have it for performance reasons.

This is part of issue #580.
Review system: move single user review data to new table

This commit adds a migration to copy all reviews that are stored in the
treenode table to the new review table. Each review gets a new entry
there. This data migration might take a while to run, if there are a lot
of reviewed treenodes. Also, it can be undone, if wanted.

This is part of issue #580.
Upgrade django-devserver to version 0.8.0

Version 0.8.0 is a proper Django 1.6 compatible release. We had to use a
Git version before this and can now specify a real release. This is much
cleaner.
Review system: make review update support multi users

This commit updates the review update backend function to write into the new
review table and therefore to support multiple users per treenode.

This is part of the work on issue #580.
Review system: update node info to support multiple reviews

This commit updates the front and backend for providing treenode information
(e.g. by pressing <I> when a treedenode is selected). It now supports multiple
reviewers per node and displays them as a list.

This is part of the work on issue #580.
Tracing statistics: remove review statistics

It is now possible that more than one user can review a tree node. The review
statistics chart would need to reflect this in some way. Because it isn't clear,
yet, how to do this and whether such a statistic is needed in the first place,
this commit removes it.

This is part of the changes discussed in issue #580.
Treenode table: simplify user dictionary creation
Style change: make treenode table backend more PEP8 compliant
Treenode table: make imports explicit
Treenode table: show all reviewers in last column

Since it is now possible to have multiple reviewers per treenode, this commit
makes the last column of the treenode table display a list of all reviewers.

This is part of the work for issue #580.
Review system: generalize review retrieving method from treenode table

This method returns a defaultdict with treenodes as keys and a list of
reviewer IDs as values. The result set can be reduced by passing a list
of treenodes or a list of skeletons. The as values listed users can be
modified by passing a mapping function (to list e.g. user names instead
of IDs). The treenode table now makes use of this function.

This is related to the changes discussed in issue #580.
Review widget: use TH elements in table header

Most browsers automatically emphasize TH elements by making their text bold.
This helps to distinguish table content from header and this commit makes use of
this fact.
Review widget: remove 'Review revisions' button and functionality

Now that there are multiple reviews per neuron possible, there is no
need anymore for resetting all revisions of a neuron. Reviews can be
done in parallel. Therefore, this commit removes this functionality
and the button.

This is part of the work discussed in issue #580.
Review widget: remove 'Review revisions by others' button and functionality

Now that there are multiple reviews per neuron possible, there is no
need anymore for resetting the revisions done by others in a neuron.
Reviews can be done in parallel. Therefore, this commit removes this
functionality and the button.

This is part of the work discussed in issue #580.
Review widget: add basic support for multi-user review

This commit updates frontend and backend code of the review widget to support
multiple users reviewing treenodes. Each treenode has now a list of reviewer IDs
attached which is empty if there are no reviews. The review table itself shows
always the current user in the first column, because this is usually what one is
interested in. The last column contains a union of the reviews of all users
having contributed to the neuron or a specific segment, respectively. If no
reviews have been done, yet, one the selected neuron, only one column (the one
for the current user) is shown and the union column is omitted.

This commit is part of the work discussed in issue #580.
Review system: make reset of own revisions support multiple users

Reviews have their own table now to support multiple users per review. This
commit makes the function to reset ones reviews aware of this.

This is part of the changes discussed in issue #580.
Review widget: highlight currently selected segment

Highlighting the current segment, makes it much easier to find in a larger table
of segments. To do this, this current text makes the text of the current text
bold.

This commit relates to issue #580.