Create a rake task to create an EE branch from a CE branch

We should create a rake task that would help creating an EE branch from a CE branch:

bundle exec rake "gitlab:dev:create_ee_branch[my-ce-branch]"

Then we could just advertise it in the ee_compat_check job output.

The next would be to automate the branch + MR creation (as a WIP if there are conflicts) from the CI job.