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/maintenance/user/api_keys_controller_test.rb

@@ -1,51 +0,0 @@
-require 'test_helper'
-
-module Maintenance
-  module User
-    class ApiKeysControllerTest < ActionDispatch::IntegrationTest
-      context "An api keys controller" do
-        setup do
-          @user = create(:gold_user, :password => "password")
-          ApiKey.generate!(@user)
-        end
-
-        context "#show" do
-          should "render" do
-            get_auth maintenance_user_api_key_path, @user, params: {user_id: @user.id}
-            assert_response :success
-          end
-        end
-
-        context "#view" do
-          context "with a correct password" do
-            should "succeed" do
-              post_auth view_maintenance_user_api_key_path(user_id: @user.id), @user, params: {user: {password: "password"}}
-              assert_response :success
-            end
-
-            should "not generate another API key if the user already has one" do
-              assert_difference("ApiKey.count", 0) do
-                post_auth view_maintenance_user_api_key_path(user_id: @user.id), @user, params: {user: {password: "password"}}
-              end
-            end
-          end
-        end
-
-        context "#update" do
-          should "regenerate the API key" do
-            old_key = @user.api_key
-            put_auth maintenance_user_api_key_path, @user, params: {user_id: @user.id, user: {password: "password"}}
-            assert_not_equal(old_key.key, @user.reload.api_key.key)
-          end
-        end
-
-        context "#destroy" do
-          should "delete the API key" do
-            delete_auth maintenance_user_api_key_path, @user, params: {user_id: @user.id, user: {password: "password"}}
-            assert_nil(@user.reload.api_key)
-          end
-        end
-      end
-    end
-  end
-end