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.

app/helpers/icon_helper.rb

@@ -144,4 +144,8 @@ module IconHelper
   def link_icon(**options)
     icon_tag("fas fa-link", **options)
   end
+
+  def plus_icon(**options)
+    icon_tag("fas fa-plus", **options)
+  end
 end