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

db/structure.sql

@@ -5796,6 +5796,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20220211075129'),
 ('20220318082614'),
 ('20220403042706'),
-('20220403220558');
+('20220403220558'),
+('20220407203236');