Use GitLab logo when showing CI pipeline status on GitHub

On the CI pipeline statuses shown on GitHub, instead of using the GitHub user avatar, use the GitLab logo:

From https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4591#note_60383339:

The logo comes from the personal access token used to set up the project. If we set this up to use either a GitHub App or GitHub OAuth I think it would use the icon belonging to that App/Oauth token.