Fix mention autocomplete for invited groups

What does this MR do and why?

Fix mention autocomplete for invited groups

This does increase the visibility of private groups if they are invited to a public group. Invite restrictions should cover this though as this doesn't expose directly who is in these groups (or their member count.)

It was also already possible to mention these groups they just didn't show up in autocomplete.

Related to #209101 (closed)

Screenshots or screen recordings

Screenshots

Before

After

How to set up and validate locally

1. Create new group
2. Invite group to another project
3. Try and mention group on issue/MR. This will work with or without the changes in this MR.
4. Leave the previously created group.
5. Try to mention group again. With MR change this will show the group in autocomplete popup. Without the changes in this MR it will not show (see screen shots above)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.