api keys: rework API key UI.

* Add an explanation of what an API key is and how to use it.
* Make it possible for the site owner to view all API keys.
* Remove the requirement to re-enter your password before you can view
  your API key (to be reworked).
* Move the API key controller from maintenance/user/api_keys_controller.rb
  to a top level controller.

test/factories/api_key.rb

@@ -0,0 +1,5 @@
+FactoryBot.define do
+  factory(:api_key) do
+    user
+  end
+end