Add and maintain listing for most active threads.

The cached queries are updated by the activity update job.

Like the other event listings, this is employee-only for now.
Remove banned threads from listings for non-admins.
Make LIVE events stand out a little more in event listings.
embed: Include <base> so relative links work.

In production, the embed is hosted off-cookie-domain on redditmedia.com.
That domain is heavily restricted in what URLs are allowed to be
accessed on it. Relative links in the liveupdate embed would end up
403ing and broken. This was especially bad for author links.
Add NSFW flag and interstitial redirect to live threads.

If a contributor flags the thread as NSFW, it will check for the user
over18 flag and redirect to the interstitial if not present.