A bunch of description fixes

* Move description and caption to both be in the bottom left
* Make empty description grey and italic
* Fix problem where description was sometimes appended twice
* Move description UI stuff into a separate file, and make it more MVCy
* Tests for description UI stuff
* General framework, to an extent, for UI classes

Change-Id: Ibc8c576cd8a41c2e3cf7e13f1b9e093384fb4655
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/120
Merge "A bunch of description fixes"