* Show the ban length instead of the ban expiration date in ban notices.
* Fix the ban notice to not say "Your account has been temporarily
banned" when it's a permanent ban.
Changes:
* Change the `expires_at` field to `duration`.
* Make moderators choose from a fixed set of standard ban lengths,
instead of allowing arbitrary ban lengths.
* List `duration` in seconds in the /bans.json API.
* Dump bans to BigQuery.
Note that some old bans have a negative duration. This is because their
expiration date was before their creation date, which is because in 2013
bans were migrated to Danbooru 2 and the original ban creation dates
were lost.
Ensure dtext is always wrapped in a `<div class="prose">` or
`<span class="prose">` (for inline dtext) container so that dtext css is
properly applied.
