Create public beta sign-up form for TB Pro

[malini]

Kinda hard to figure out which repo to file this in since we don't have a unified repo for all things tb.pro or for the tb.pro website. Filed this here since eventually we'll put these users into accounts.tb.pro when we're ready to go live with public beta.

Anyway, we need to put up a beta sign-up page on tb.pro. More details are here: https://www.notion.so/mzthunderbird/tb-pro-page-for-signups-17e2df5d45ae80a0878df2dc41d0c604#17e2df5d45ae8066bfffc8040a2d5292

We need to create a double-opt-in MailChimp form and host it on https://tb.pro/. For functionality, we just need an email input form and a "join the waitlist" submit button. We'll get input from Design and Marcomm next week, but we can start work on the list.

[malini]

main requirements for MailChimp are: 1) allow us to gather users email addresses and allow them to validate the email addresses through MailChimp, and 2) allow us to email those validated users with an email inviting them into the beta.

For 2) could you find out if we can batch emails out instead of letting everyone in at once, so I only let in like 20% of the list at a time, and if MailChimp keeps track of those we reached out to so we don't accidentally invite them again?

[radishmouse]

Update: in MailChimp, audience ID a508104cf8 is set to double opt-in:

[radishmouse]

This commit (already merged via #13) redirects user to the MailChimp-hosted form (using a MailChimp URL from .env)

Will open a new PR with custom form after input from Design and Marcomm.