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.