[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>

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.

Also, fixes broken specs

- Remove extra method for authorize_admin_project
- Ensure project presence
- Rename 'read_repo' to 'read_repository' to be more verbose

Also fixes spec failures on presenter and docs

[ci skip]

Update index.md to describe leaving out the host in prometheus['listen_address'] to allow public access.

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