Replace vendored difflib library npm dependency

[maurizi]

Description

As part of porting the password validation strategy from Temperate, we copied over a vendored & modified version of difflib, a Javscript port of a Python utility used by Django's password validation.

We'd prefer to not include a vendored library for this, and we don't particularly need to use difflib like we did in Temperate, but password validation is used on both the frontend & backend and until #179 is fixed we can't import any npm dependencies into the shared module.

AC

• The vendored difflib module is removed, and replaced with an equivalent dependency from NPM