SideBySide2: Add shift-m to mark reviewed and open next file

In the old UI "M" marked the reviewed flag (if not already set)
and moved to the next file in the change. Implement a similar
behavior in the new SideBySide2 UI.

Bind as "shift m" instead of "M" due to vim mode using lowercase
"m" as a mark character, which some users may actually use to make
bookmarks during a review.

Change-Id: I1e7780e6265fa3380aae09c034583dec7ec4830a
2 files changed