Enable RSS feeds for (release) tags

This helps a lot in monitoring projects like apps, libraries, etc.

see e.g. https://github.com/pineconellc/angular-foundation/tags.atom

They should be publicly visible (i.e. without requiring an account)

This is an extension of my feature request gitlab-foss#2629 (closed)

Proposal

• Add an RSS button next to the New Tag button on /<group>/<project>/tags
• Button should open .atom feed of tags