[Fork] After cloning without write access, fork and publish from Desktop

[kuychaco]

Extracted from #8509 (Problem 1) for tracking work on this particular issue

Problem: Trying to push changes to a repo you don't have write access to

If you've cloned a repo and make some changes with the intent to contribute, you're basically stuck. You can't push those changes to the repo you've cloned because you don't have write access, and you can't bring them over to a fork that doesn't yet exist in a low-friction way.

Desktop could let you know you don't have write access and give you a nice pathway to fork and create a PR. This is conceptually similar to local commits to protected branches, where ideally we wouldn't allow you to do that (because you'd never be able to push those commits to that branch), and instead give you a nice path to somewhere where your work can eventually be pushed without issue.

A potential solution is outlined in #3918 by @say25.