Add critical flow / "smoke" tests in Cypress

[aitchiss]

Since introducing Cypress E2E testing, we've largely added tests as we introduce new features, and we haven't really taken stock of what our critical flows are. For example, we don't have any E2E tests for the onboarding flow, despite this arguably being one of the most important user flows in the app.

I'm creating this issue more as a 'suggestion', and whoever takes it forward can perhaps lead a discussion within the engineering team of how we want to do things, but I would suggest we create a folder of "smoke tests" - i.e. the minimal set of tests that give us early indication of something critical being broken (e.g. user sign up, sign in, post creation).

This would help give us confidence in refactors, but we could also consider in the future whether we want to only run the smoke tests on each PR build (to speed up build times).

[github-actions]

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