Make tag API for release feature consistent

Make tags API consistent with other tags methods. This changes the endpoint from `PUT /projects/:id/repository/:tag/release` to `PUT /projects/:id/repository/tags/:tag_name/release`.

On thing the API is still missing, is an error if the tag does not exist. Right now it returns 200 even the tag does not exist. I'll fix that such it returns 404.

@stanhu Can you review?
@rspeicher This MR should go into 8.2

See merge request !1864