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.
Statistics widget: remove review pie chart headline

Since the review pie chart itself is gone, there is no need for the headline
anymore.
Improve performance of statistics queries

To refer to all reviews of a certain project, requires to JOIN either the
skeletons or the treenodes (class_instance table or treenode table). Before, the
treenode table was used, but it turns out that using the class_instance table is
usually faster.