Support Project Avatar removal in REST API

This change set adds support to remove project avatars using the REST API, much like it's already possible for the Topics API.

To support this I've changed the POST | PUT /projects endpoint to be handled by workhorse. Can someone with more experience and the big picture verify this particular part of the change? Is this something which makes sense? Is it implemented correctly?

If this is the case and all looks good with the Projects API here, I'll also implement similar changes to the other Avatarable endpoints, like Groups and Users.

This change was driven by a request in the terraform provider.

/cc @nagyv-gitlab @nmezzopera can you help me triage this?