Implement the ability for instances to define a list of disallowed hashtags (#7176)

The goal here isn't to prevent these hashtags from existing, but just to strongly curtail their usage; The hashtags may still exist in the database via federated status, or from being created prior to this feature.

config/locales/en.yml

@@ -684,6 +684,9 @@ en:
         one: "%{count} video"
         other: "%{count} videos"
     content_warning: 'Content warning: %{warning}'
+    disallowed_hashtags:
+      one: 'contained a disallowed hashtag: %{tags}'
+      other: 'contained the disallowed hashtags: %{tags}'
     open_in_web: Open in web
     over_character_limit: character limit of %{max} exceeded
     pin_errors:

config/settings.yml

@@ -47,6 +47,7 @@ defaults: &defaults
     - root
     - webmaster
     - administrator
+  disallowed_hashtags: # space separated string or list of hashtags without the hash
   bootstrap_timeline_accounts: ''
   activity_api_enabled: true
   peers_api_enabled: true