Add ability to mark tags as deprecated

* Deprecated tags can't be added to posts, but existing deprecated tags
  in a post won't be removed
* Only empty tags can be marked as deprecated manually
* No tags can be manually undeprecated
** These limits don't apply to admins
* Deprecating or undeprecating a tag will create a new mod action to
  prevent people from going rogue
* Added deprecate/undeprecate commands for BURs
* Deprecating a tag via BUR removes all implications to and from it as well

app/models/mod_action.rb

@@ -60,6 +60,8 @@ class ModAction < ApplicationRecord
     tag_implication_create: 140,
     tag_implication_update: 141, # XXX unused
     tag_implication_delete: 142,
+    tag_deprecate: 240,
+    tag_undeprecate: 242,
     ip_ban_create: 160,
     ip_ban_delete: 162,
     ip_ban_undelete: 163,