Improve ability to leave a project

Related to #3342 (closed).

At some point during our redesign the ability to leave a project became much harder. !2166 (merged) will restore the only current option for a non-master/owner to leave a project. However, this is at the bottom of the project page, below the README. This is really obscure. In earlier versions a user could click 'Leave' from their project list.

1. Can we find a more prominent place on the project page for the access info and leave link?
2. Can we add a 'leave' button on the project list again, or elsewhere?

cc/ @dzaporozhets What do you think?