Enable adding draft on active line

Modified CodeMirrorDemo to highlight active line.

Added KeyMap from 'c' to a handler that adds drafts on active line,
edits existing drafts, or replies to published comments.

Modified lineOnOther() in LineMapper to return both the line number
and whether the line on the other side is aligned with the one on
this side. Added tests to verify.

Change-Id: I732ded8d76163a6b4dc76fcd45dbd9233090ce9c
11 files changed