feature: user management (login/logout)

[romanc]

Description

This PR adds basic user management (sign up, log in, log out, password reset, email change, account deletion). This works "out of the box" with mix phoenix auth gen. Mail server setup and config was a charm as always... In the end, I've extended the generated code such that users can delete their (own) account. In addition

• a bit of HTML work on the home page,
• a simple privacy policy,
• and an FAQ page

---

Remaining things to do:

• wire up mailserver to send emails for user registration
• setup staging instance to test deployments / releases (e.g. mail server setup)
  • staging database
  • runner service to run release candidates on port 4004
  • staging server rc.cklist.org (dns entry and certificate update)
    Bonus points for adding a https://www.cklist.org -> https://cklist.org redirect
  • staging server nginx config
• Test mailserver config (login / pw-reset email) in staging environment
  • Fixed issue with migrate script on deployment
  • Changed adapter to Swoosh.Adapter.*
  • Configure either SMTP or Mua to send emails via mail.cklist.org
• Prune GHA secrets (e.g. no need for prod DB)
• Prune dependencies. We use Swoosh SMTP Adapter now, so we need this and gen_smtp and don't need Mua and its dependencies.
• update email templates and sender in lib/cklist/accounts/user_notifier.ex
• new tests: describe "about page" / "contains current app version" / "renders faq"
• simple privacy policy (needed since people can login now and we thus store personal information)
• fix login tests (can't find user's email address anymore, which is in the generated html template)
• allow users to delete their account (from settings)
  • functionality && manual testing
  • automated tests

Checklist

Guess what, we use checklists too 😉

• I did a self-review of the proposed changes
• I commented my code, particularly in hard-to-understand areas
• I added automated tests for new functionality (if applicable)