Make group developer-level members maintainers on projects they create

This is a follow-up on gitlab-ce#44959.

Problem to solve

Developer-level group members may now (%12.0) create projects within a group. This is wonderful, and helps a lot, thanks!

There is however one problem: they only get the same rights as they inherit from the group. Therefore, if they:

• Didn't set the correct permissions/visibility level/CI/integrations
• Didn't set the correct project name or settings
• Created the project by mistake and want to delete it

Then they still need to ask the group members with elevated permissions, therefore making the design goal of gitlab-ce#44959 only partially reached.

Proposal

I propose to make developer-level group members maintainers on the projects they create within the group namespace.

I believe that matches the intended aim of the original change, however it does mean a change in the default permissions model.