MPP-3495: Fix next URL check

[jwhitlock]

The function is_safe_url in privaterelay.allauth.AccountAdapter checks if the provided next URL is safe. This was recently customized, but is broken.

For example, this URL:

https://stage.fxprivaterelay.nonprod.cloudops.mozgcp.net/accounts/fxa/login/?next=profile_refresh

will redirect the user after login to:

https://stage.fxprivaterelay.nonprod.cloudops.mozgcp.net/accounts/fxa/login/profile_refresh

Most URLs are not URL names, so most URLs will be identified as non-safe, and the user will redirect to /accounts/profile/?.

This PR fixes the function and improves it so that it will also redirect to next.js pages. Some things to try:

• Redirect to email test view:
  • Login URL: http://127.0.0.1:8000/accounts/fxa/login/?process=login&next=/emails/wrapped_email_test
  • Redirects to: http://127.0.0.1:8000/emails/wrapped_email_test
• Redirect to FAQ page:
  • Login URL: http://127.0.0.1:8000/accounts/fxa/login/?process=login&next=/faq/
  • Redirects to: http://127.0.0.1:8000/faq/
• Redirect to phones page with full URL:
  • Login URL: http://127.0.0.1:8000/accounts/fxa/login/?process=login&next=http://127.0.0.1:8000/phone/
  • Redirects to: http://127.0.0.1:8000/phone/
• Fail to Redirect to non-existent page
  • Login URL: http://127.0.0.1:8000/accounts/fxa/login/?process=login&next=/404-page
  • Redirects to: http://127.0.0.1:8000/accounts/profile/?
• Fail to Redirect to other domain
  • Login URL: http://127.0.0.1:8000/accounts/fxa/login/?process=login&next=https://monitor.mozilla.org/
  • Redirects to: http://127.0.0.1:8000/accounts/profile/?
• Next URL after logout
  • Logout URL: http://127.0.0.1:8000/accounts/logout/?next=/faq/
  • Redirects to: http://127.0.0.1:8000/faq/

This issue was re-discovered while working on MPP-3698, which failed type checking with mypy --check-untyped-defs privaterelay/allauth.py. It now passes.