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.

db/migrate/20210310221248_change_expires_at_to_duration_on_bans.rb

@@ -0,0 +1,11 @@
+class ChangeExpiresAtToDurationOnBans < ActiveRecord::Migration[6.1]
+  def up
+    change_column :bans, :expires_at, :interval, using: "expires_at - created_at"
+    rename_column :bans, :expires_at, :duration
+  end
+
+  def down
+    change_column :bans, :duration, :timestamp, using: "created_at + duration"
+    rename_column :bans, :duration, :expires_at
+  end
+end