Global logout logic

We need to expire all of a user's tokens when they change or reset their password:

• Add a nullable tokens_invalid_before DateTime column to the User model
• Create migration (then verify the behavior in editor, and strip out auto-generated comments):

  $ make shell-db
  root@hash:/code# alembic revision --autogenerate -m "Short description here"
  

• Add logic to the token verification logic to check if the user has a tokens_invalid_before timestamp set, and if so ensure that the token's "issued at" timestamp is after that point (otherwise reject the token)
• Update the code logic for password resets and changing passwords to invalidate tokens issued prior to the time the new password is set
• Add tests to bring coverage to 100% for the new code

Start by locating the User model, token verification logic, and password reset and password-change handlers. Use make shell-db, then inspect the Alembic autogeneration and migration output. Done means tokens issued before either password operation are rejected, newer tokens continue to work, and tests provide 100% coverage for the new logic.