Allow packages for larger ecosystems be maintained by teams

There are larger ecosystems which cannot easily be maintained by a single maintainer. For example gnome, which is currently maintained (on paper) by @Cogitri, but we recently added 2 new developers to @team/gnome.

The current policy is that packages should have a single maintainer. The reason behind this is that ultimately, someone should feel responsible for the packages, and we want to avoid the bystander syndrome (someone else will fix it).

The proposal is to allow only for packages part of larger ecosystems (desktop environments, toolchains, etc), for which we typically already have a team/* group created. This would mean the maintainer entry would probably list something like team/gnome as name. I'm not sure what would make sense to use as e-mail address, either something fictional, or we could provide forwards (team-gnome@alpinelinux.org).

@Cogitri, @Leo: this would also mean aports-qa-bot would need to be adjusted to deal with this. As you cannot assign merge requests / issues to groups, it should be assigned to one member of the group. This could either be some designated member, or a round-robin assignment. In any case, the team should be pinged ('CC: @team/gnome').