Update frontend engineer job description

Created by: felixfbecker

This updates our frontend engineer job description to reflect our current needs more: A little less emphasis on advanced TypeScript frontend code, and more emphasis on UI engineering, React and CSS.

I also added a "What you will do" section because I want to give candidates an idea of what their day-to-day looks like and that it's not implementing designs thrown over the fence by a design team, but engaging work where we (and expect them to) closely collaborate with design and product to engineer UIs, and we want to attract candidates that will flourish in that role.

Also explainaing the new coding exercise that will come with it to be transparent about what we will expect them to be able to accomplish in the interview process.