restrict individual approvers to project members

What does this MR do?

Only show users with access to the project in the Approvers dropdown

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

not sure the best place to add tests. Some options:

• make selects_helper_spec.rb for users_select_tag
• tests for approvers_select and move MR page to use that

Why was this MR needed?

Screenshots (if relevant)

User 1 without access

Settings page:

MR Page:

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #1593 (closed)