Add support of group handles when filtering by assignee or author in IssuableFinder

Problem

Please see Problem section here

Summary

As part of #468693 we need to add support of group handles while filtering by assignee or author. That will allow organization managers to set up their boards\VSA\lists with specific teams filters.

Technical details / acceptance criteria

1. There should be limit in number of users mentioned by group tag (e.g. 100 users max)
2. There should be access checks so only users\group tags accessible to current_user will be expanded to actual list of users.
3. We should limit the query to direct members on the currently queried group, not including members in sub-groups
4. Private group handles shouldn't be exposed to users without permission
5. Update docs to mention exactly how this works.

Reach out to someone from grouporganizations via @mandrewsgl for review on MRs.