Add `web_url` to branch endpoint response

What does this MR do?

Contributes to: #214547 (closed)

It adds web_url to the API response for a branch endpoint.

Reasons for the change

1. We want to extend Static Site Editor functionality (see #214547 (comment 328423508))
2. Consistency. Merge requests, commits, users APIs expose web_url already, but branch API does not.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers