[batch] Redesign batch front end pages

[daniel-goldstein]

This revamps the batch.front_end web pages to be a bit less cluttered and more usable. I brought in tailwindcss instead of using our existing sass. I mainly find it easier to work with but am fine porting it to sass if anyone feels strongly against changing css tools. In order to avoid overhauling all our HTML in one PR, I added a flag to the jinja context use_tailwind that is currently only on for the batch service. layout.html then uses this flag to determine which CSS and header it should use. If this gets to a state we're happy to merge, it should be a lot less work to bring over auth and CI and delete that flag.