Create reusable form to upload avatar

Currently we have 3 places were we upload avatars:

1. User Avatar
2. Groups Avatar
3. Project Avatar

Each has its own haml file and js file. User avatar allows cropping, the other 2 don't.

HTML and requirements are the same between all them, we can create a reusable haml file and a reusable js - with cropping - for all of them.

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16092#note_52606080