Completely recreated user management

I'm not sure there's much else useful to say besides that. I totally
recreated the whole user management system, because the previous system
was (poorly) done in JavaScript only, and it was an unmaintainable mess.
For example, it seems that crispy forms were at one point going to be
used, but the auto-generated HTML ended up being hard-coded into
base.html, which also contained an obscene amount of user management
logic.

I tried to keep the user management at feature parity, with some
improvements here and there; for example, changing passwords and emails
is now implemented and works fully, as does registration (including
email confirmation). However, there were things I scrapped (such as
having user addresses). Nonetheless, I feel the current user code is
much, much, much more maintainable, and these features should be
relatively easy to add in the future.

Best of all, all of this code works without JavaScript enabled. However,
if you do have JavaScript enabled, Bootstrap will automagically warn
people of common mistakes (such as inputting an invalid email). Also,
errors are displayed cleanly thanks to crispy forms. These libraries
were already installed ... but apparently underused.

Caveat: Order history is (still) not implemented.