Add shortcuts for adding users to a project team with a specific role

This also updates some specs to use these new methods, just to serve as an example for others going forward, but by no means is this exhaustive.

Original implementations at !5992 and !6012.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/20944

EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7565.

Reassigned to @rspeicher

Enabled an automatic merge when the build for 7b3c018e succeeds

MWBS set!

Reassigned to @smcgivern

Canceled the automatic merge

Added 839 commits:

• 7b3c018e...1f42fd41 - 838 commits from branch master
• f7ace65e - Add shortcuts for adding users to a project team with a specific role

Compare with previous version

Enabled an automatic merge when the build for f7ace65e succeeds

Status changed to merged

Mentioned in commit b3c6bea4