Add validation reports for pull requests

[dempseyatgithub]

Currently a malformed field in an SE proposal causes a fair amount of thrash across two repositories:

• Missing or incorrect data presented on SE Dashboard due to issues in the proposal
• Issue reported to swift-org-website
• Issue investigated and pull request created and submitted to swift-evolution
• PR reviewed and merged into swift-evolution
• Issue in swift-org-website closed

Since this tool already performs validation during metadata extraction, it would be useful to extend it to generate a validation report for PRs and used as a PR check for the swift-evolution repository.

This would eliminate the thrash by ensuring proposals are already correct before they are merged.

The exact requirements of the validation report, how this tool would integrate, etc. are yet to be determined.

[dempseyatgithub]

One note is that proposals are typically added as pull requests when they are still being pitched, so it will be important to recognize proposals that should not be completely validated.

Some things, like having a title, should always be present, but something like a valid Review Manager won't be present possibly even after an 'on stage' status like Awaiting Review is assigned.

A strong goal would be to avoid causing thrash for folks managing proposal merges due to overstrict validation in cases like this.