Add a more comprehensive list of available Django password hashers.

Review Request #10544 — Created April 27, 2019 and submitted — Latest diff uploaded

Review Board

Modern versions of Django provide new password hashers, and deprecate
legacy ones. Some of those deprecated ones are still used in production,
so we can't get rid of them just yet. This change sets a list of all the
hashers we know we'll want to use.

Note that Django will only use the first one for creating new hashes,
but will support passwords made in all following hashers.

Unit tests pass on Django 1.6 and 1.11.