Mix commit notes with merge request notes on MR show page

Created by: riyad

general:

• add ids to diff lines (for linking, see below)
• add helpers for dealing with notes for different targets

on MR show page

• show merge request notes together with all the notes from its commits
• if a note is from a commit/diff line add a link to the commit/line
• fix vote counting to only count merge request notes (not commit notes)

Depends on #1629 Fixes #1622 (closed)

screen shot: