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/functional/application_controller_test.rb

@@ -39,7 +39,7 @@ class ApplicationControllerTest < ActionDispatch::IntegrationTest
     context "on api authentication" do
       setup do
         @user = create(:user, password: "password")
-        @api_key = ApiKey.generate!(@user)
+        @api_key = create(:api_key, user: @user)
 
         ActionController::Base.allow_forgery_protection = true
       end