[API] Fix URLs in the 'Link' header for 'GET /projects/:id/repository/contributors' when no value is passed for 'order_by' or 'sort'

Signed-off-by: Rémy Coutable <remy@rymai.me>

Previous one returned HTTP 404. It was discussed in http://gitlab.com/gitlab-com/marketing/general/issues/1966
that it is preffered to use /press based git cheat sheet pdf because it
is opened in browser's pdf viewer by default rather than downloaded.

Request Japanese proofreader permissions

Mentions https://gitlab.com/gitlab-org/gitlab-ee/issues/3768

This is useful for annotated tags, where the deferenced target is not the same as the tag object. At the moment there is no way to differentiate the two through the tag API.

This change adds a "target" property and leaves the existing "commit" property alone so that existing behaviour is not altered.

- Added unmergeable action
- Added directly_addressed action

Also, fixes broken specs