Self-service Password Change: should use a toast to indicate Password success

[reivilibre]

see the designs in #2148

Currently we use a workaround where we just show a success message and then expect you to click back.

Introduced in: #2863

[matrixbot]

For your information, this issue has been copied over to the Element fork of matrix-authentication-service: element-hq/matrix-authentication-service#2865