Numerous issues with anonymous users registering accounts on Drupal 8 (especially from contribution pages)

We've recently discovered numerous issues with anonymous users registering accounts on Drupal 8. This affects both the standard Drupal 8 registration form, but especially registering a new account on contribution forms. The problems span both civicrm-core and civicrm-drupal.

Issues:

• Registration on /user/register will work after cache clear, but not again ("Error type: Could not find a valid session key.")
• On /user/register, validation errors on CiviCRM profiles will cause a 500 error
• Checking for username/email availability doesn't work correctly
• On contribution pages, registering a new account will error out when trying to display the "Thank you" page and execution of the request stops early before it can finish everything. The issues following this one only appear after fixing this point, because they are things later in the process...
• On contribution pages, user accounts are always created "Blocked" (even when users have permission to register their own accounts)
• On contribution pages, user's aren't fully logged in (they get "CiviCRM logged in" but not "Drupal logged in") after registering an account (even when users would normally be logged in immediately after registration per Drupal settings)

We worked through code changes for all this stuff - I'll share the PRs shortly!