Improve performance of GroupsController#show

What does this MR do?

This MR includes various performance improvements for GroupsController#show, such as:

1. Reducing the number of queries used to see if a user is a member of a group
2. Adding an index to speed up counting the number of group merge requests

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/43048