I2I: Convert Promise chains to async/await

From my manual perusal of different tests, as well as some of the issue threads linked in https://github.com/ampproject/amphtml/issues/14360, I see there are a lot of issues with functions that create promise chains with no `catch` handler, ensuring that failures will be unhandled rejections and making it impossible to catch them synchronously in test code. I suspect general confusion around how async functions, promises, and async errors work all combine to create a systemic issue where there's no consistent "contract" that a developer or test-writer can depend on when dealing with functions that use or produce promises.

I think a general cleanup effort to migrate from Promises toward async/await could clear up a lot of confusion and make some of these issues either go away or easier to diagnose. There are tools that can do this somewhat automatically (ex. [VS Code supports Promise->async](https://umaar.com/dev-tips/182-typescript-async-await/), and there are [tools](https://www.npmjs.com/package/@codemodsquad/asyncify) to do this at scale) and this is something that could be done in bulk, with small no-op PRs pushed out Rosie-style to individual files and their owners, distributing the workload across the team. This could occur in tandem with a Deep Dive providing clarity and best practices for the team.

Link for convenience: https://caniuse.com/#search=await

@rsimha WDYT?
@jridgewell Would shifting to async/await carry performance or compilation issues we should be concerned about, or does Babel do a decent job with this already?