SideBySide2: Hide reviewed checkbox if not signed in

Users cannot mark a file as reviewed unless they are signed in, as the
server needs the user identity to store the reviewed flag. The checkbox
is hidden on ChangeScreen2 for anonymous users, so hide it in SBS2.

Change-Id: I96605222fab5ab0cb64c157b51dc4ca92ec4732d
SideBySide2: Do not offer 'r' keybinding to anonymous users

Anonymous users cannot mark a file as reviewed, as the server
has no way to track the stored flag. Disable the keybinding
so it does not appear in the help dialog.

Change-Id: I415a24754d1b4b5120b5b40e074ee697cf921b52