reddit/reddit-plugin-liveupdate

In production, liveupdate embeds are hosted on a off-cookie-domain. This
domain would also fail the is_reddit_url test which would mean that the base
URL wouldn't be set as secure even if the client connection was secure. This
would in turn cause javascript to not execute because it was loaded
insecurely.

This requires renaming our ContributorTableItem because when it gets a
JSON representation it starts conflicting with r2's.  In general, the
view models in plugins should stay prefixed to avoid this kind of mess
so this is just good practice anyway.

This reverts commit 4be8d44.

Previously, it was possible to create contributors without inviting
first or to add an invite for an already contributing user because this
endpoint didn't check for existence before setting the permissions level
which due to the way the data model works is the same as adding an
object.

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

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.

The cached queries are updated by the activity update job.

Like the other event listings, this is employee-only for now.

This just shows the linked update for now without surrounding updates
for context.

* get rid of some unnecessary nesting
* pull out various stuff into variables
* make the selector for the header more obvious

The (low length limit) description field will be useful for embeds and
for summarizing the thread. Resources will become collapsable
eventually.

(It's totally a cromulent word.)

This does a few things:

* takes advantage of the new unsandboxed version of mediaembeds from
  r2 to remove one unnecessary level of <iframe>s.
* styles the embed to look more like a regular old self post.
* applies the current subreddit's stylesheet to the embed to allow
  the embed to be in-theme for a subreddit.