Atom feed: Replace `summary` with `content`

The body/content of Atom (RSS) feed entries seems to be empty. Atom feed entries should contain:

• the comment contents
• the bug report contents
• the commit comment, and its summary
• a link to the project (e.g. if somebody commits)
• a link to the author (e.g. of commits, issues, ...)
• a link to the created object (e.g. commit hash, issue, merge request, branch, ...)

And the feed contains no icon.

Thanks