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/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