Projects: custom avatar improvements

[agjohnson]

Right now, projects disconnected from a remote repo get a generated avatar and projects connected through a remote repo get the social account avatar. Most of the time the remote repo avatars look great, especially if repositories have differing owners, but it can lead to redundant avatars.

A couple improvements here:

• GitLab supports project avatars, we can use these if we aren't already
• GitHub doesn't but support repo avatars but does have repo "social preview" images. I don't know if these images are in the existing API response or not, or only usable with an HTTP request
• We could follow Allow configuration of organization (user?) profile pictures without gravatar #94 and just allow manual project avatars too

[humitos]

This is related to #94