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.

db/migrate/20171021191900_move_keyword_mutes_into_glitch_namespace.rb

@@ -0,0 +1,7 @@
+class MoveKeywordMutesIntoGlitchNamespace < ActiveRecord::Migration[5.1]
+  def change
+    safety_assured do
+      rename_table :keyword_mutes, :glitch_keyword_mutes
+    end
+  end
+end