Custom favicons not being replicated by Geo

Noted in 11.0, testing failover for the GCP migration, we saw retrying ~Geo jobs with this message in sidekiq:

NameError: uninitialized constant Gitlab::Geo::FaviconUploader

This means that without action, when we fail GitLab.com over for the GCP Migration, any custom favicons will not be replicated, but will be lost. Is that a problem? If it's an instance-level setting, I think we could probably live with it and just restore them post-failover.

At the very least, we should document this in the Geo limitations section of the documentation. We should also fix it, either for %11.0 or %11.1

MR introducing custom favicon functionality is https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14497 and https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5999/diffs . It doesn't look like the Geo case was considered during development.

Object storage

Also make sure migration to object storage generates an event (if gitlab-org/gitlab-ee!5966 doesn't resolve that already)

/cc @akaemmerle @dawsmith @ash.mckenzie @DouweM