Open mirror settings to the API

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Main discussion

Right now there are a number of mirror specific attributes that are exposed in the API project entity but they are not able to be set/updated through the API

Attributes:

• mirror
• import_url (right now is only available on create params => https://gitlab.com/gitlab-org/gitlab-ee/blob/master/lib/api/projects.rb#L77 we should add it to update as well)
• username_only_import_url
• mirror_user_id
• mirror_trigger_builds
• only_mirror_protected_branches
• mirror_overwrites_diverged_branches

Bonus:

Add attributes for remote mirrors as well

• id
• url
• enabled

\cc @DouweM