title: Tagging like github or gitlab
body: <p>I want to introduce github like tagging in my app, where a repo has some predefined tags/labels and only those tags can be used on issues/PRs. Project can <code>have_many issues</code>. Tags are related to both repos and issues, as in issues <code>has_many tags</code> and repos <code>has_many tags</code>. I can't seem to figure out how to implement it. I am using <a href="https://github.com/mbleigh/acts-as-taggable-on" rel="nofollow">acts-as-taggable-on</a>. So, I initialize the tag_list of repo when it is first created with:</p>
  
  <pre><code>repo.tag_list = 'bug, feature, improvement, inspiration, discussion, help'
  </code></pre>
  
  <p>I am allowing user to add tags at the same time issue is created (ie on new form of issues). Now how do I validate in issue model that tags being added is in the tag_list of the repo on which issue is being created?</p>