Upgrade Django libs

[cmc333333]

Only tweak that was needed was to add the overextends tag to the template settings.

Warning The docker dev container may complain about a version mismatch. That'd be due to the checked out versions of -site and -core requiring an incompatible version of Django. If you run into this, there are two solutions:

• delete the regulations-site and regulations-core directories in eregs_libs and try again
• update your checkouts to 7.1.0/3.1.0 (respectively) or master

It'd definitely be worth trying to structure the -site/-core checkouts differently to avoid that "minimum version" problem. See #482 for a potential solution.

[cmc333333]

Thanks @tadhg-ohiggins ! I rebased to account for the submodule approach in #484