Add password strength meter and validation on the change password page #866

As raised by @megoth in #842

Not directly related, but more of a note to self: We've added repeat password on another page, we probably should do that here as well.

But for this case, we need to find a good markup of the password component. I've used two divs with col-md-3in the previous design, so might revert something a bit here, unless you have a better suggestion?

Originally posted by @megoth in #842