Support GitLab username namespace communities

Problem to solve

When you create a project under your personal GitLab namespace, it doesn't have a group (just a namespace). So people can create Gitter rooms for their personal projects, we need to support Gitter communities backed by personal namespaces

Proposal

For the backend, this would look like GL_USER_SAME similar to our existing GH_USER_SAME

For avatars we can use the stable endpoint: https://docs.gitlab.com/ee/api/avatar.html#get-a-single-avatar-url

Links / references

Additional work

• Zendesk: Associate tymonx community with @tymonx GitLab user namespace.