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/models/glitch.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+module Glitch
+  def self.table_name_prefix
+    'glitch_'
+  end
+end