ChangeScreen2: Show on which categories a reviewer can vote

Old change screen had grey styled cells in approval table for labels a
reviewer hadn't right to vote on. The new change screen uses chips to
present the reviewers name and therefore this valuable information is lost.

Add it back in form of tooltip with the list of votable labels for each
reviewer.

Bug: issue 2078
Change-Id: I0f101e43c625bfe1f181b46574c6677d930a5c19
6 files changed