Add Continuous Releasability Contract

Created by: beyang

This adds a page to the handbook defining a Continuous Releasability Contract, which holds that (1) master should always be releasable to customers and (2) we use feature flags for every new feature.

One side effect of abiding by the Releasability Contract is that end-of-cycle QA should become a formality, rather than a period during which we expect to catch a non-zero number of release-blocking issues. In order for this to happen, certain teams may have to do a more thorough QA pass before merging/enabling new features in master than they have in the past.

I believe this is just a written statement of our existing policy w.r.t to PRs and the state of master branch. However, since that policy was not written down and our team has grown since we decided on it, I am not sure that every teammate is aware of it. I'd therefore like everyone on the engineering team to sign off on this.

Initial reviewers, please add your teammates as reviewers after you approve this PR.

Original discussion in RFC 86. This is in service of OKR 1.vi.c.a ("Major/minor and patch releases take 1 hour (including general QA but excluding customer-specific testing")