Adapt history statistics function to multi user review

This method isn't used right now. However, the adjustments were't too
involved and should give one at least the right direction when the
method should be used again.

This is part of the chages discussed in issue #580.
Treenode table: deactivate sorting of reviewer column

Since it isn't clear, yet, how sorting of the reviewers column should work in
the treenode table when there are multiple reviewers per node, sorting is
disabled for now. All reviewers of a node are listed in this column, though.
This change has been made in the course of removing all references to the
reviewer_id field of the treenode table to be to remove it soon.

This change is part of the discussion in issue #580.
Add helper method to retrieve review count per user in a skeleton
Move review status calculation to review module

This functionality was previously only needed by the graph widget, but is also
useful in other contexts. It is therefore moved into its own function.

This is part of the discussion in issue #580.
Tree utils: prevent unwanted change in length of list of extra fields

The props list contains the names of user definable fields that are queried in
addition to some standard fields. The length of this list is used to map them to
the results returned by the database. However, before this list got overridden
in every iteration and since its length was changed to include the standard
fields, the length was wrong in the next iteration. This commit fixes this by
using a separate variable for the resulting mapping. This leaves the original
props list untouched.
Make user evaluation multi user review aware

To make the user evaluation admin tool support multi user reviews, this commit
makes it use always the newest review on a node to build epochs. Currently, all
reviews are taken into account, but this will be changed to make sure now "own"
reviews are included for the user being evaluated.

This change is part of the discussion in issue #580.
Support user exclusion when querying review status

This allows one to query the review status of skeletons while ignoring reviews
of a set of certain users.
User analytics: support new review system

This is commit makes the user analytics code get the review information from the
new review table instead from the treenode table.

This change is part of issue #580.
Remove all reviewer_id and review_time occurrences from test data

All reviewer IDs have been -1, i.e. "not reviewed", anyway. So no new data had
to be created. This is part of the changes discussed in issue #580.
Adapt objects module to new multi review system

This is part of the changes discussed in issue #580.
Fix small example wrt. new review system
Remove reviewer_id and review_time fields from location based tables

With the new review system, these columns are now obsolete in the
treenode and the connector table. This commit removes this columns,
so make sure you have a backup!

The migration removes the fields only from the location table, because
the connector and the treenode table inherit it (as specified in the
initial migration). They will therefore also not have both fields
anymore after applying the migration

This is part of issue #580.