need to reset user passwords (+UI)

[William Reade]

When a subtly malicious admin creates a new user and gives them a .jenv to identify them, the admin could keep a copy and use it to impersonate that user in future; when a client first logs in it should be required to reset the password and record a fresh one before taking any action.

This will involve factoring out and reusing existing logic currently used by agents.

The reset-password functionality must also be exposed in the UI, so that a user can respond effectively if their password is compromised.