Self-service Password Change: support the state of not having a password if passwords are enabled

[matrixbot]

This issue was originally created by @reivilibre at matrix-org/matrix-authentication-service#2862.

On the Account Management page:
We probably want to add has_password or something to the User node in GraphQL — or maybe something a bit more carefully designed¹ — so we can show

On the password change page:
We presumably want to allow setting a password if you don't already have one(?), but this flow doesn't exist right now...

This issue really only matters when we support instances with both SSO and/or passwords in a hybrid setup.

Footnotes

1. e.g. we might want to expose the password version, or the time it was last updated, etc ↩