urls: Support sha256-based tokens

urls: Support sha256-based tokens

Django 3.1 changed the default hashing algorithm used for things like
password reset tokens from SHA-1 to SHA-256. As noted in the release
notes [1], this is configurable via the 'DEFAULT_HASHING_ALGORITHM'
transitional setting, but that's only intended to allow upgrades of
multiple instances in a HA deployment and shouldn't be used post
upgrade. Instead, we need to fix our URLs to support the longer tokens
generated by SHA-256.

Long term, we want to replace these regex-based routes with the simpler
flask-style template string routes. That's not really backportable so
we'll do that separately.

[1] https://docs.djangoproject.com/en/3.1/releases/3.1/#default-hashing-algorithm-settings

Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes: #394