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.
Review widget: add follower checkboxes to select next segment

This commit introduces a new feature to the review widget: Every reviewer column
gets now a checkbox added in front of the user name. If checked, one "follows"
this user. This only has an effect for selecting the next segment (e.g. by
pressing 'E') during/after the review of one segment. The behavior up to now was
that the next segment is selected, that hasn't been completely reviewed by
oneself. This is now extended to have the next segment selected that has not
been completely reviewed by any followed reviewer. By default this is only
oneself, so default net effect is the same as before. It can, however be
extended to express for instance the trust in the reviews of another person by
skipping segments reviewed by her/him and continuing with a segment that has not
been completely reviewed by any followed user.

This change is part of the review system update discussed in issue #580.