Move KeywordMute into Glitch namespace.

There are two motivations for this:

1. It looks like we're going to add other features that require
   server-side storage (e.g. user notes).

2. Namespacing glitchsoc modifications is a good idea anyway: even if we
   do not end up doing (1), if upstream introduces a keyword-mute feature
   that also uses a "KeywordMute" model, we can avoid some merge
   conflicts this way and work on the more interesting task of
   choosing which implementation to use.

app/lib/feed_manager.rb

@@ -138,7 +138,7 @@ class FeedManager
   end
 
   def filter_from_home?(status, receiver_id)
-    return true if KeywordMute.matcher_for(receiver_id) =~ status.text
+    return true if Glitch::KeywordMute.matcher_for(receiver_id) =~ status.text
 
     return false if receiver_id == status.account_id
     return true  if status.reply? && (status.in_reply_to_id.nil? || status.in_reply_to_account_id.nil?)