Support adding gitlab.com repos on demand in sourcegraph.com
Created by: tsenart
This change set adds support for adding gitlab.com repos on demand in our sourcegraph.com environment, similarly to what we already do for github.com.
Test plan: Manual testing + automated tests
Release notes:
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We need to add a Gitlab external service with a url=gitlab.com and an access token from some shared sourcegraph gitlab.com account before merging this.