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.

app/models/keyword_mute.rb

@@ -0,0 +1,13 @@
+# == Schema Information
+#
+# Table name: keyword_mutes
+#
+#  id         :integer          not null, primary key
+#  account_id :integer          not null
+#  keyword    :string           not null
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+
+class KeywordMute < ApplicationRecord
+end