Added test for flow from invite to access of project private

[alneberg]

... and fixed the final little piece for inviting researchers.

Before submitting a pr:

• Tests passing
• Black formatting
• Migrations for any changes to the database schema
• Rebase/merge the dev branch
• Note in the CHANGELOG

[codecov]

Codecov Report

Merging #912 (a62c39c) into invite-researchers (3bd7102) will increase coverage by 0.26%.
The diff coverage is n/a.

@@                  Coverage Diff                   @@
##           invite-researchers     #912      +/-   ##
======================================================
+ Coverage               85.92%   86.18%   +0.26%     
======================================================
  Files                      26       26              
  Lines                    2800     2795       -5     
======================================================
+ Hits                     2406     2409       +3     
+ Misses                    394      386       -8     

Impacted Files	Coverage Δ
dds_web/api/user.py	88.28% <ø> (ø)
dds_web/security/auth.py	98.11% <0.00%> (+1.16%)	⬆️
dds_web/api/schemas/user_schemas.py	89.41% <0.00%> (+7.05%)	⬆️

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