Introduce the gr-diff-mode-selector which componentizes the diff mode
buttons already used in the file list and adds them to the diff view.
With this new component, when authenticated users change their diff
preference using the new selector (or by the 'm' keyboard shortcut), the
new mode is saved to their preferences. Unauthenticated users see no
change in diff mode preference persistence.
The diff selector is now consistently labeled as "Diff view" rather than
as "Diff views".
Bug: Issue 8144
Change-Id: I4b30714deb9a466e707b3d4ae90c1d4c60222c64
