Improve candidate account creation process

[bryli]

Currently, automatically creating candidate accounts requires 1) manually downloading the semester's candidate form Google Sheets, and 2) separating out names into First and Last name, and 3) creating CSVs separated into 10 candidates per file with 4) the columns "First name", "Last name", and "Berkeley email" (case-sensitive).

This is sort of annoying and requires the use of an external script, so the flow essentially is -> locate the candidate Google Sheets, manually download it -> go to bash terminal or something to run a script -> upload each CSV one at a time.

We want to improve parts of this process, such as allowing a single CSV to be uploaded for all candidates. Ideally this would be a one-click process by copy pasting the candidate sheet, but that is a more involved process.

[ochan1]

Such a feature to do all at once was created
#384
#445

Was removed for some reason in #464

It just used a background task to create all the candidates so it doesn't hold up the website thread and timeout, and uses JS to check on progress

Recommendations when reviving the feature: Fix the email pipeline, and try to see if there can be a way to stop the thread (not just script in case of hangups)