Merge initial work from Pull Request Builder GSOC

[ericholscher]

This is mostly an extension of #5750. It moves all the ongoing and reviewed work from the team into this branch, which is now ready for merging into master when we see fit.

[humitos]

I'd like to know how I can test this locally. I tried using ngrok but for some reason PR are not triggered --I may need to do something else with my testing GH app as well.

@saadmk11 can you guide me with the steps I need to do locally?

[saadmk11]

To test This Locally we need to check few things:

1. Use ngrok to run the local server.
2. Setup allauth for github and add a test OAuth app on github.
3. Update the webhook url on github repository to add the url generated by ngrok.
4. Add ENABLE_EXTERNAL_VERSION_BUILD feature flag to the project.

That should get you up and running 💯

[ericholscher]

Going to merge this to let it simmer in master for a few days, then ship it 🎉