[WIP] check if username given is in User tables

This is a simple change that checks if the given username is in the User table as well, along with the corresponding test. This fixes https://github.com/learningequality/ka-lite/issues/375. The change is added to the line where the FacilityUser table is checked for the same username.

This is marked as WIP since this doesn't checking for conflicts when syncing.