Cog icon & mw.ui styling for expand button

Includes some small refactoring of the bootstrap code to open
MediaViewer so it can be used in a more controller-like fashion, and
removing the .mw-mmv-filepage-menu class which was added in the parent
commit but had no effect since the module was not loaded by default.

Change-Id: I2e7405e694af96e8eca4fcc839b60306232ead01
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/719
Merge "Cog icon & mw.ui styling for expand button"