Add eslint back to project

[seankwilliams]

Need to either get off of react-dates (which isn't compatible w/React 17 + 18) or wait until react-dates is compatible in order to get back to eslint. (Currently, we need to build the project with npm i --legacy-peer-deps, which causes issues with eslint due to multiple React versions -- I think. Possible there's some way to get eslint working with the current setup.)

See eslint files removed in 8b1ea83