Add KeywordMute model.

Gist of the proposed keyword mute implementation:

Keyword mutes are represented server-side as one keyword per record.
For each account, there exists a keyword regex that is generated as one
big alternation of all keywords.  This regex is cached (in Redis, I
guess) so we can quickly get it when filtering in FeedManager.

spec/models/keyword_mute_spec.rb

@@ -0,0 +1,5 @@
+require 'rails_helper'
+
+RSpec.describe KeywordMute, type: :model do
+  pending "add some examples to (or delete) #{__FILE__}"
+end