WIP: Resolve '"Add Gitter room" on project page'

What does this MR do?

Makes Gitter a first class citizen on GitLab.com by adding an "Add Gitter room" button on the project overview page.

What are the relevant issue numbers?

Closes #51830 (moved)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the style guides