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/fabricators/keyword_mute_fabricator.rb

@@ -0,0 +1,2 @@
+Fabricator(:keyword_mute) do
+end