Add Links to Branches in Calendar Activity

What does this MR do?

Add Link to Branches in Calendar Activity (Contributions for ) view (#26287 (closed)). Branches that do not exist will just be displayed as strong without a link.

Are there points in the code the reviewer needs to double check?

Is there another way to check for branch existence or is this the correct way? I also could not find any tests for the calendar events, do I have to add some and if I do, where do I add them?

Why was this MR needed?

See #26287 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #26287 (closed)