BURs: don't log mod actions for aliases/implications/mass updates.

Don't log mod actions when aliases, implications, or mass updates are
processed.

Originally aliases and implications were logged because they could be
approved outside of a BUR. Mass updates could also be performed by mods
without making a forum request. This is no longer the case.

They were also logged for debugging purposes. This is no longer needed.
This generated a lot of spam in the mod action logs when a large BUR was
approved.

app/models/mod_action.rb

@@ -41,13 +41,13 @@ class ModAction < ApplicationRecord
     forum_post_update: 101,
     forum_post_delete: 102,
     tag_alias_create: 120,
-    tag_alias_update: 121,
+    tag_alias_update: 121, # XXX unused
     tag_implication_create: 140,
-    tag_implication_update: 141,
+    tag_implication_update: 141, # XXX unused
     ip_ban_create: 160,
     ip_ban_delete: 162,
     ip_ban_undelete: 163,
-    mass_update: 1000,
+    mass_update: 1000, # XXX unused
     bulk_revert: 1001, # XXX unused
     other: 2000
   }