Enable users to change password

[merowin]

It would be nice if users can change their password.

One user told me they forgot their password, and after the session ran out, they created a new account. They've also forgotten that accounts password, but the session for that account hasn't run out yet.

This could also have synergy with #224 - if admin users can reset a users password (hypothetically), to help the user log in again, then the user can change their password to something memorable again.

[benjaminpjones]

I think this might also go hand-in-hand with email notifications as well. We could do the "forgot password" thing