Show recommended content in /explore

Shows a mix of content from:
- subreddits recommended for the user (based on subscriptions and multis)
- rising threads
- items from discovery-focused subreddits

Listing items emphasize the subreddit name and have feedback controls.

The AccountSRPrefs class builds a user preferences model on-the-fly from
subscriptions, multireddits, and a record of recent user feedback.

The AccountSRFeedback column family stores a user's recent interactions with
the recommendation UI. For example, it records which srs the user dismissed
as uninteresting, and keeps track of which srs were recommended recently to
make sure we don't show the same ones too often.

Each type of feedback has a ttl after which it disappears from the db.
Remove sr_discovery from example.ini

Internal promotions can now be run as house ads, so this variable is no
longer needed.
Not all dialects are in need of translators

This change causes language dialects to not be
marked with a (*) on the prefs page if the base,
generic language is sufficiently translated.