Ease repository deletion #3730

I hate deleting repositories because at least 5 clicks + selections, copy-paste, scroll + password.

I think this can be simplified, at the very least for forks. For example, if there are:

• no stars
• no open issues
• no local PRs
• no cross-fork PRs (via show-open-prs-of-forks)

We could show a Delete fork tab next to the Settings tab, or in more-dropdown

Such link would:

1. show a full-page 5-second deletion countdown ("click anywhere to stop")
2. call the API
3. load github.com
   • or, if the tab is in the background by then and if possible, we could close the tab once it's done (via browser.tabs.remove())