Mention other Groups with Project Access
Description
As a user I want to mention other groups with group access and not single members of the group, to notify all group members.
Proposal
Add other groups beside parent and own groups to the mention autocompletion.
Overview
What is it?
Why should someone use this feature?
- It's handy to mention groups instead of single users and sometimes needed for communication between departments.
 
What is the underlying (business) problem?
- I always have to know who is the exact person to talk to, but sometimes you don't know, but you know which department is it.
 
How do you use this feature?
- In any style of comments (issues, milestones, merge requests).
 
Use cases
Who is this for? Provide one or more use cases.
- Every User should be able to use it.
 
Feature checklist
Make sure these are completed before closing the issue, with a link to the relevant commit.
- 
Feature assurance  - 
Documentation  - 
Added to features.yml