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.

db/migrate/20171009222537_create_keyword_mutes.rb

@@ -0,0 +1,11 @@
+class CreateKeywordMutes < ActiveRecord::Migration[5.1]
+  def change
+    create_table :keyword_mutes do |t|
+      t.references :account, null: false
+      t.string :keyword, null: false
+      t.timestamps
+    end
+
+    add_foreign_key :keyword_mutes, :accounts, on_delete: :cascade
+  end
+end