FIX: If a group is unmentionable, don't render it as mentionable

Now if a group is visible but unmentionable, users can search for it
when composing by typing with `@`, but it will be rendered without the
grey background color.

It will also no longer pop up a JIT warning saying "You are about to
mention X people" because the group will not be mentioned.

app/controllers/users_controller.rb

@@ -308,7 +308,7 @@ class UsersController < ApplicationController
     groups = Group.where(name: usernames).pluck(:name)
     mentionable_groups =
       if current_user
-        Group.mentionable(current_user)
+        Group.mentionable(current_user, include_public: false)
           .where(name: usernames)
           .pluck(:name, :user_count)
           .map do |name, user_count|