Select Project/Group Avatar from URL or image in a Project

Select Project/Group Avatar from URL in addition to selection in browser.

The feature you propose is threefold:

1. Add group avatar via URL
2. Add project avatar via URL
3. Add project avatar via file in the repo. (already implemented by having a logo.png (or jpg or gif) on your master branch. See this merge request gitlab-ce!304.)

This would also make it easier to set the avatar through the API. Just pass the URL of the image, and GitLab does the rest.