GitLab and related ports maintained by me: pass maintainership to Matthias Fechner

Matthias has proven himself as capable and enthusiastic enough to update
www/gitlab on its own - which frees much of my time. :)

I wrote an update guide for him and everybody interested in it:
http://gitlab.toco-domains.de/FreeBSD/GitLab-docu/blob/master/update/how-to-update-the-freebsd-port.md