Use ! instead of # for merge requests in activity RSS/Atom feeds

Summary

When someone merged a merge request, the corresponding atom/RSS feed item for /dashboard/projects.atom or /<group>/<project>.atom has a headline like

John Doe accepted merge request #id: title

But merge requests are commonly enumerated with ! and not #.

Steps to reproduce

Merge a merge request and look at https://gitlab.com/dashboard/projects.atom or https://gitlab.com/gitlab-org/gitlab-ce.atom.

Example Project

There was no merged merge request in my dashboard activity history on gitlab.com, but it also happens here: https://gitlab.com/gitlab-org/gitlab-ce.atom.

What is the current bug behavior?

# will be used.

What is the expected correct behavior?

! should be used.

Relevant logs and/or screenshots

From my private GitLab instance:

<entry>
  <id>tag:xxx,2017-08-10:4741</id>
  <link href="https://xxx/docker/cronie/merge_requests/7"/>
  <title>xxx accepted merge request #7: Deploy Fedora 26 at docker / cronie</title>
  <updated>2017-08-10T03:00:04+02:00</updated>
  <media:thumbnail width="40" height="40" url="https://xxx/uploads/-/system/user/avatar/4/avatar.png"/>
  <author>
    <username>sedrubal</username>
    <name>xxx</name>
    <email>xxx</email>
  </author>
  <summary type="xhtml">
<div xmlns="http://www.w3.org/1999/xhtml">

</div>
  </summary>
</entry>

Output of checks

This bug happens on GitLab.com

Possible fixes

Replace # with ! in the corresponding template. IMHO should be very easy.