Connectivity widget: be more conservative with review percentage display

Up to now, the review percentage was rounded up and down for being displayed in
the connectivity widget. This commit changes it to always cut the decimals
without rounding. This prevents showing 100% by accident.

This change is part of the discussion in issue #580.
Connected partners: calculate union review count properly

The former SQL query counted all reviews for a specific skeleton, regardless if
one treenode was reviewed multiple times. This means there could have been more
union reviews for a skeleton than there are treenodes. Now it counts each
treenode only once. This code is used by the connectivity widget.

Part of the discussion in issue #580.