bans: change expires_at field to duration.

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.

test/factories/ban.rb

@@ -3,6 +3,6 @@ FactoryBot.define do
     banner :factory => :admin_user
     user
     reason {FFaker::Lorem.words.join(" ")}
-    duration {60}
+    duration { 1.week }
   end
 end