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/unit/ban_test.rb

@@ -84,26 +84,6 @@ class BanTest < ActiveSupport::TestCase
         CurrentUser.user = nil
         CurrentUser.ip_addr = nil
       end
-
-      context "when only expired bans exist" do
-        setup do
-          @ban = FactoryBot.create(:ban, :user => @user, :banner => @admin, :duration => -1)
-        end
-
-        should "not return expired bans" do
-          assert(!Ban.is_banned?(@user))
-        end
-      end
-
-      context "when active bans still exist" do
-        setup do
-          @ban = FactoryBot.create(:ban, :user => @user, :banner => @admin, :duration => 1)
-        end
-
-        should "return active bans" do
-          assert(Ban.is_banned?(@user))
-        end
-      end
     end
   end
 end