WIP: start converting user management views' tests to pytest format

[rebeccacremona]

This work was started during the Q3 housekeeping cycle, but was not finished.

Still to do:

• test_org_export_user_list
• test_sponsored_user_export_user_list
• test_create_and_delete_user
• All the tests in "### ADDING NEW USERS TO ORGANIZATIONS ###"
• All the tests in "### ADDING EXISTING USERS TO ORGANIZATIONS ###"
• All the tests in "### VOLUNTARILY LEAVING ORGANIZATIONS ###"
• All the tests in "### REMOVING USERS FROM ORGANIZATIONS ###"
• All the tests in "### ADDING NEW USERS TO REGISTRARS AS SPONSORED USERS ###"
• All the tests in "### ADDING EXISTING USERS TO REGISTRARS AS SPONSORED USERS ###"
• All the tests in "### TOGGLING THE STATUS OF SPONSORSHIPS ###"
• All the tests in "### ADDING NEW USERS TO REGISTRARS AS REGISTRAR USERS) ###"
• All the tests in "### ADDING EXISTING USERS TO REGISTRARS ###"
• All the tests in "### REMOVING REGISTRAR USERS FROM REGISTRARS ###"
• All the tests in "### ADDING NEW USERS AS ADMINS ###"
• All the tests in "### ADDING EXISTING USERS AS ADMINS ###"
• All the tests in "### REMOVING USERS AS ADMINS ###"

Not in scope:

• Any tests of filters. those are going to be a pain to convert, as long as JSON fixtures are still being installed. (edit: I added a helper to delete interfering JSON fixtures... but I still think those should all be done at once... may be there is a much more efficient way to test)
• "### SIGNUP" (should be their own file)
• "### PASSWORD RESETS ###" (should be elsewhere)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.19%. Comparing base (ffa57fa) to head (45f914d).
Report is 35 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3621      +/-   ##
===========================================
+ Coverage    69.67%   70.19%   +0.51%     
===========================================
  Files           54       54              
  Lines         7661     7801     +140     
===========================================
+ Hits          5338     5476     +138     
- Misses        2323     2325       +2     

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