Add frontend code linting via Prettier

[matthinz]

Bring in Prettier to format frontend code.

From the updated README:

Automatic linting and formatting

Linting and formatting is wired up using Husky and lint-staged. Running yarn install will install a git pre-commit hook that will lint and format code in the commit using the following tools:

Files	Formatter / linter
Ruby (.rb)	Standard
CSS / SCSS (.css, .scss)	Prettier
Javascript (.js, .jsx)	Prettier
Terraform (.tf)	terraform fmt