Split diffviewer models into multiple files
Review Request #9597 - Created Feb. 7, 2018 and submitted
diffviewer/models.pyfile was getting quite large (~750 lines) and
is going to be growing due to some new model additions for DVCS support.
It was about time it got cleaned up.
This change also cleans up some documentation to bring it in line with
our current standards.
Ran unit tests
Revision 2 (+419 -287)
Checks run (2 succeeded)