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.
* Fix it so that all edit forms show an error banner if the form
has validation errors. Previously forms had to manually call
`error_messages_for`, which not all forms did.
* Fix it so that the full validation error message is shown next to each
input attribute that had errors. Also update the styling of these
error messages to look better.
