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/unit/api_key_test.rb

@@ -3,14 +3,8 @@ require 'test_helper'
 class ApiKeyTest < ActiveSupport::TestCase
   context "in all cases a user" do
     setup do
-      @user = FactoryBot.create(:gold_user, :name => "abcdef")
-      @api_key = ApiKey.generate!(@user)
-    end
-
-    should "regenerate the key" do
-      assert_changes(-> { @api_key.key }) do
-        @api_key.regenerate!
-      end
+      @user = create(:user)
+      @api_key = create(:api_key, user: @user)
     end
 
     should "generate a unique key" do