WIP: Add support for surfacing a link to the branch or tag name in push messages

What does this MR do?

In this MR, we append the branch_link to Chat message push notification activity titles. This allows services like MS Teams to display push messages with meaningful links to the Git branch or tag name.

Note to reviewers:

1. To add this feature, I updated the private message() helper in push_message.rb so it:

• Can be reused by activity method as message(title_only=true)
• Refactored message formats so users see the same action messages with or without markdown enabled

2. push_message_spec.rb unit tests updated to reflect updated activity message format. Added an additional spec to verify deletion of tag names.

What are the relevant issue numbers?

Closes #51959 (closed)

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
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.