Issue Summary

It looks like the change in #3261 doesn't handle the case of existing users that haven't accepted their invitations yet, so users trying to sign up using invitations created in a previous version incorrectly get the "This invitation has already been accepted" message. https://github.com/getredash/redash/pull/3229/files#diff-a0c5448134fdb627ae48d25bad76393fR40

I was able to fix this in our org with:

UPDATE users
SET details = '{"is_invitation_pending": true}'::json
WHERE password_hash IS NULL

Maybe this case should be handled in the migration here? https://github.com/getredash/redash/blob/master/migrations/versions/e7f8a917aa8e_add_user_details_json_column.py

Steps to Reproduce

1. Send invitation from a version prior to Invitation Pending changes #3229 and Fix invitation pending for existing users #3261
2. Upgrade to current master
3. Try to access the invitation link

Any other info e.g. Why do you consider this to be a bug? What did you expect to happen instead?

Technical details:

• Redash Version:
• Browser/OS:
• How did you install Redash: