Set up /settings/keyword_mutes. #164.

This should eventually be accessible via the API and the web frontend,
but I find it easier to set up an editing interface using Rails
templates and the like.  We can always take it out if it turns out we
don't need it.

config/locales/en.yml

@@ -491,6 +491,7 @@ en:
     export: Data export
     followers: Authorized followers
     import: Import
+    keyword_mutes: Muted keywords
     notifications: Notifications
     preferences: Preferences
     settings: Settings