Separate sign-up views into their own module

[cmsetzer]

Following on @rebeccacremona's work in #3607, this breaks the sign-up views off of user_management.py and puts them into their own module, user_sign_up.py.

There are some functions such as approve_pending_registrar that fall somewhere in between "user management" and "sign-up" in terms of functionality; in such cases, I did my best to minimize import dependencies between modules and keep functions alongside their associated views. Open to feedback though.

[codecov]

Codecov Report

Attention: Patch coverage is 95.55556% with 10 lines in your changes missing coverage. Please review.

Project coverage is 69.26%. Comparing base (02beba5) to head (f7a73ce).
Report is 7 commits behind head on develop.

Files with missing lines	Patch %	Lines
perma_web/perma/views/user_sign_up.py	95.26%	10 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3610      +/-   ##
===========================================
+ Coverage    69.20%   69.26%   +0.05%     
===========================================
  Files           53       54       +1     
  Lines         7193     7207      +14     
===========================================
+ Hits          4978     4992      +14     
  Misses        2215     2215              

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[rebeccacremona]

Yes! Into it!

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ SMTPServerDisconnected: please run connect() first /sign-up/ View Issue

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