Resolve "Scope users by group inside dropdowns if I'm on group page (Issue, MRs)"

What does this MR do?

This retrieve the current group ID in the user dropdowns on the group's issuable list page so that only the group's members are returned in the filtering dropdowns (author and assignee).

Are there points in the code the reviewer needs to double check?

The data-group-id is already set on the search filter input (https://gitlab.com/gitlab-org/gitlab-ce/blob/46bdcd64056e8e5ed8d397b0ad5accaa7e5e9d06/app/helpers/search_helper.rb#L150-152), the only missing part was to retrieve it on the JS side.

Why was this MR needed?

To make the scoping of users consistent between the project's issuable list page and the group's issuable list page, as noted here: https://gitlab.com/gitlab-org/gitlab-ce/issues/20824#note_40119351.

Screenshots

For the following group members:

Before	After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #20824 (closed)