Enable prettier for the `amp-github-apps` repo

[rsimha]

For consistency, let's use the same .prettierrc as amphtml.

[rsimha]

@rcebulko Reopening this since the code doesn't actually follow the format recommended by prettier. (You'd need to run eslint with --fix and change the prettier entry in .eslintrc from a warning to an error.)

[rcebulko]

@rsimha On the Prettier PR @erwinmombay suggested making it a warning rather than an error. I'll spin up a linted PR with the changes and loop you both in as reviewers.