Support inter-repository pull requets

[Uncommon]

Building on #360, where PRs are shown if they are from an existing remote

• [ ] In the remote sidebar item, when there are pull requests, add a button that indicates how many there are
• [ ] The button opens a popover showing the list of pull requests
• [ ] If a PR's branch is already downloaded, have the option to navigate to the branch
• [ ] Have the option to add the PR's repository as a remote if it isn't already