Allow autocomplete and mentions for all GitLab groups

New proposal: allow you to autocomplete all groups when mentioning (would be better to also allow mentioning any user).

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Old proposal: This basically means putting an abstraction above @-references like @all.

I would be nice to define notification-subgroups on project level, like @developers, @marketing, @ci-people. Adding access rights for each notification subgroup would be also interesting feature (who can use @-references and in which context).

I wasn't able to find feature proposal like that in tracker :)