Add E2E tests for Facebook login

[patcon]

Re-ticketed from https://github.com/pol-is/polis/pull/527 https://github.com/pol-is/polis/pull/523 (🎉 @patrickas )

It seems like there are some issues with Facebook login flows, that are hard to sort. I'd like to start adding E2E Cypress tests to ensure these flows aren't breaking.

Current blocks to allowing this:

• [x] Lack of HTTPS in docker environment (prevents interacting with FB app) https://github.com/pol-is/polis/pull/540
• [x] Lack of easily-testable email functionality (SES is required to send email and complete login) https://github.com/pol-is/polis/pull/433

Resources

• Facebook API Explorer. For exploring API while writing tests.
• Facebook API Reference: Test Users. For creating and managing test/fake users programmatically. (fake users can have permissions added/revoked, or made to be friends with one another)
• Facebook Docs: Test Users. Navigating UI for managing test users.

[patcon]

A contributor has recently added Facebook Login support to this Cypress plugin that supports third-party login support: https://github.com/lirantal/cypress-social-logins

(normally, cypress is not designed to work within urls outside the site domain)

EDIT: working through how to get this going https://github.com/lirantal/cypress-social-logins/pull/67

[patcon]

As noted in https://github.com/compdemocracy/polis/issues/1549#issue-1408752578, it's perhaps good to also test facebook login (and social login generally) both within iframes (embedded) and directly on the site. It seems that social login for embedded convos might break when it otherwise works.