Resolve "<link href=""> and <a href=""> not properly filled in activity RSS feed"

What does this MR do?

This MR changes links inside RSS feed elements from path to uri for comments, pushes and creation of projects.
This makes it possible to go directly from your RSS reader to comments, new projects or commit shas.

What are the relevant issue numbers?

gitlab-org/gitlab-ce#50823

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #50823 (closed)