Establish practices to avoid messy stacked PRs and rebases

[nick-gorman]

In the lead up to the initial release for APSRC we ended up with very large PRs, multiple stacked PRs, and blocking where one PR couldn't be merged until another one was done. It would be good to establish a set of practices to avoid this situation in the future. The goal would be to adopt a work flow which:

• keeps PRs small so they are easier and faster to review
• avoids or helps manage stacked PRs (to avoid messy rebases)

Some approaches to talk through (noting we might adopt multiple):

• Synchronous reviews: jump on a call together to get a small PR done quickly
• Feature flags: hide new features behined flag checks so you can merge in work in progress code
• git rebase --onto or tools like Graphite.dev to manage stacked branches
• Interface definitions to allow work on different features in parallel

[nick-gorman]

• Agreed to try the feature flag approach with frequent small PRs.
• Happy to try synchronous reviews
• #TODO Nick to document