Add collection-based landing page that shows all users' collections (paginated)

[srudolph]

This feature would add a mediagoblin.ini configuration variable named 'most_recent_media_view' that, by default, shows the main landing page the way it is always shown now: with a paginated and reverse chronological list of media entries. It could be changed to 'Collection' to instead show a reverse chronological list of non-empty collections.

For now, a random media entry from the collection will be shown to represent the collection. In the future, this could be improved to allow the collection's creator to select a key entry to always show.

Some benefits of this change:
-Default behavior remains as-is
-Discoverability of collections is enhanced
-Allows increased customizability

[srudolph]

Implementation