FEATURE: track date api key was last used

Start tracking the date an api key was last used. This has already been
the case for user_api_keys.

This information can provide us with the ability to automatically expire
unused api keys after N days.

app/models/api_key.rb

@@ -35,6 +35,7 @@ end
 #  updated_at    :datetime         not null
 #  allowed_ips   :inet             is an Array
 #  hidden        :boolean          default(FALSE), not null
+#  last_used_at  :datetime
 #
 # Indexes
 #