Allow Owners to transfer projects between namespaces

Description

Allow for changing of project namespaces via the API

Proposal

I believe that if the UI allows a user to perform an action, they should be able to do so via the API for parity. This doesn't seem to be the case for changing project namespaces -- using GitLab.com for example. This is useful for when you want to move many repositories in bulk.

Links / references

See https://gitlab.com/gitlab-com/support-forum/issues/396#note_66966507 -- @markglenfletcher told me to open the issue here.

/label ~"feature proposal"