Improve automated builds

[cwillisf]

Proposed Changes

• Support non-signed builds for PRs from forks
• Support automated macOS builds
• Determine whether or not to sign the build based on the branch
• Add a script to generate self-signed SSL certificates and use it for non-signed CI builds
  • This script is also useful for local development
• Share CircleCI config between Windows and macOS where possible

Reason for Changes

This should bring Scratch Link's automated builds to a fully functional state. Specifically:

• We will automatically build any branch in LLK/scratch-link as well as pull requests targeted at any branch in LLK/scratch-link
• The branches develop, main, and master are treated specially:
  • These builds will use proper, official SSL certificates
  • The application and installer will be digitally signed
• In other branches, including PRs:
  • These builds will use self-signed "mock" SSL certificates
  • The application and installer will not be digitally signed