Make it easier to find invited group members

We had a number of team members struggle to find invited members of a
group. Searching for the e-mail address did not work because search only
works with members with user accounts.

This commit changes two things:

1. Breaks out the invited members into a separate table.
2. Adds search capability for the invited members.

This is an EE port of:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28436