feat: add a pr list page

[gitcommitshow]

• List of open PRs
• Group by user
• Group by repo
• Make the external contribution identification more robust

[gitcommitshow]

Deterministic external user permission check returns error for repos where the app is not installed. This will lead to missing the PRs from the list which were external contribution in a repo where app was not installed.
This is not essential requirement but it will be good to have if we can also list those PRs.
Possible solutions

1. If deterministic permission check throws error, show those PRs in a different list with "get head repo" button. On clicking the button, request the full PR details, which should finally confirm whether the PR is an external PR or not.