user invites: exclude existing users based on name, not just email

Created by: slimsag

Multiple people internally have reported that user invites on the homepage / sign up page shows users already on Sourcegraph.

We exclude users from the suggestions if anyone has an account with that verified email.

I expect this is worse for Sourcegraph employees: we mix-and-match personal and professional emails between what we commit to Git with and what we sign up with on Sourcegraph.com accounts.

The backend could be altered to:

1. Add a DB index to lookup users by name
2. Check if a user with that name already exists, in addition to the current email check.