Check to make sure that passwords match before creating or updating an account

This addresses #1974. 

This was introduced a while back in a fix for #1838 I believe. In an attempt to not check the password when we were editing a user that had already been created, we stopped checking when they were just being created as well. [see more deets](https://github.com/learningequality/ka-lite/blame/release-0.12.0/kalite/facility/forms.py)

Anyway, the change below does not make sense to me anymore, but it did 5-10 minutes ago. In my testing this appears to work. I recommend testing the following scenarios (in each case, trying to create a user without the passwords matching, and in the case of editing, trying to mismatch passwords AND trying to edit details without changing passwords)

* As an admin
   * Create a student 
   * Edit student details 
* Logged out
   * Try signing up
* logged in as student
   * try editing details by clicking on "Change Password" from user page.

In my quick testing, this worked. @aronasorman if you don't have time/energy to do these tests, I will do them again tomorrow just to be sure. Currently my brain is like :egg: :laughing: :shaved_ice: :soccer: