[Request access] Refactor "Approve" access request to use an atomic transaction

[erzhtor]

Currently "Approve" access request button handler uses three GQL queries to check if the username is free, to approve the access request, and to create a new user. This should be rewritten to use a single GQL mutation/resolver. The "create user" part must be reused with the existing "CreateUser" mutation.

/cc @sourcegraph/source/iam