ip bans: add hit counter, deleted flag, new ban type.

* Make IP bans soft deletable.
* Add a hit counter to track how many times an IP ban has blocked someone.
* Add a last hit timestamp to track when the IP ban last blocked someone.
* Add a new type of IP ban, the signup ban. Signup bans restrict new
  signups from editing anything until they've verified their email
  address.

app/models/mod_action.rb

@@ -48,6 +48,7 @@ class ModAction < ApplicationRecord
     tag_implication_update: 141,
     ip_ban_create: 160,
     ip_ban_delete: 162,
+    ip_ban_undelete: 163,
     mass_update: 1000,
     bulk_revert: 1001, # XXX unused
     other: 2000