UX: Add description to the 2FA page when adding new admins (#16098)

This PR adds an extra description to the 2FA page when granting a user admin access. It also introduces a general system for adding customized descriptions that can be used by future actions.

(Follow-up to dd6ec65061)

lib/second_factor/actions/grant_admin.rb

@@ -9,9 +9,14 @@ module SecondFactor::Actions
 
     def second_factor_auth_required!(params)
       user = find_user(params[:user_id])
+      description = I18n.t(
+        "second_factor_auth.actions.grant_admin.description",
+        username: "@#{user.username}"
+      )
       {
         callback_params: { user_id: user.id },
-        redirect_path: admin_user_show_path(id: user.id, username: user.username)
+        redirect_path: admin_user_show_path(id: user.id, username: user.username),
+        description: description
       }
     end
 

lib/second_factor/auth_manager.rb

@@ -52,6 +52,9 @@ the following methods:
   redirected to after the action is finished. When this key is omitted, the
   redirect path is set to the homepage (/).
 
+  :description => optional action-specific description message that's shown on
+  the 2FA page.
+
   After this method is called, the auth manager will send a 403 response with a
   JSON body. It does that by raising an exception that's then rescued by a
   `rescue_from` handler. The JSON response contains a challenge nonce which the
@@ -169,6 +172,9 @@ class SecondFactor::AuthManager
       allowed_methods: allowed_methods.to_a,
       generated_at: Time.zone.now.to_i
     }
+    if config[:description]
+      challenge[:description] = config[:description]
+    end
     secure_session["current_second_factor_auth_challenge"] = challenge.to_json
     nonce
   end