Formalizing committers: Now using CODEOWNERS file

[Tristan Van Berkom <tr...@codethink.co.uk>]

Hi all,

    As per recent announcements regarding formalizing the project
committers[0], we have now setup the GitLab CODEOWNERS file to reflect
these changes.

Changes made:

  * All project members are now "Developers" and only the initial 4
    committers are "Maintainers".

    Of course this means Developers can still run pipelines on branches
    within our own CI and the same easy policy "just ask and ya shall
    receive" regarding requesting Developer access remains unchanged.

  * Only "Maintainers" can push and merge to protected branches, with
    the exception of Marge, who is also allowed to merge.

  * Added new .gitlab/CODEOWNERS file listing the main committer
    usernames with blanket access to the entire repository.

  * Merge requests now require one approval from a code owner.

This approach allows us to continue using the Marge bot while still
requiring approval from a maintainer.

In the new workflow for submitting branches, basically just get your
code approved by a code owner, and then the merge request can be
assigned to marge and it should end up merged (possibly after a
rebase).


Cheers,
    -Tristan

[0]: https://mail.gnome.org/archives/buildstream-list/2020-May/msg00023.html