Tiny tweaks to form processing

[rebeccacremona]

This PR makes just a few tweaks to the how the new form for adding many users to an organization at a time saves data.

• it adjusts when if commit is checked, so that all the logic except for the actual write operations is run if commit is False

• it creates new OrganizationAffiliation objects all together in one batch, instead of two batches (one for brand new users and one for existing users)

• when retrieving the collection of existing affiliation objects, it uses select_related('user'), so that later, when running set(affiliation.user for affiliation in preexisting_affiliations), each call to affiliation.user does not trigger a separate call to the database

• it adjusts how we handle potentially mixed/uppercase and cast-to-lowercase email addresses so that a) duplicate emails are detected in the CSV even if they differ in casing, and b) we can retrieve the raw email address from self.user_data

I took the liberty of renaming a few variables, adding a few comments, and slightly changing the order of operations, so that existing users are handled first, and then we move on to new users.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 69.67%. Comparing base (344f09b) to head (5478649).
Report is 68 commits behind head on develop.

Files with missing lines	Patch %	Lines
perma_web/perma/forms.py	91.66%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3673      +/-   ##
===========================================
+ Coverage    69.05%   69.67%   +0.62%     
===========================================
  Files           54       54              
  Lines         7493     7661     +168     
===========================================
+ Hits          5174     5338     +164     
- Misses        2319     2323       +4     

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[cmsetzer]

Looks good to me. I left a few comments about minor points of coding style; feel free to adopt or ignore as you prefer.