Add ability to export CSV of applicants

[kevin-zou]

Description

The code for this should already exist in the old hacker assessment code, so you should have a baseline to go off of. The exported list should be the list of applicants currently visible in the applicant list component. This means that any search queries/filters that have been applied should remain even in the exported CSV (shouldn't be too bad since we already keep state on the filtered applicants).

Frontend redesign

You can see how this button was previously rendered by running Admin locally and going to localhost:3000/assessment, then clicking on the "Export" button. A modal should pop up with two buttons (one to export CSV and another to download resumes).

Right now the modal is a custom component, but it would be great if we could align it with the rest of Admin (ie. using our Modal component).

Where it should be

For the time being we can put this as a third icon in the header of the applicant list in Eval:

Clicking this button should open a modal, where a user can click a button to download the CSV.

[kevin-zou]

@naijwu I pushed up some initial work I did onto branch csv-export (https://github.com/nwplus/admin/tree/csv-export)

Basically it adapts to this year's schema for the Applicant object in Firebase. Feel free to continue working off this branch or just take its changes and work on your own branch! Let me know if you have any questions, and thanks for taking this again <3