Fix flaky voucher redemption Cypress test

[Splines]

I was able to reproduce a flaky Cypress test locally. Fixed by adding cy.then(() => {...}); statements after logging out, then logging in again as a new user. I also intercept a POST request now to make sure the deletion of the account is finished before we continue.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.27%. Comparing base (6382fb5) to head (1d80266).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #726      +/-   ##
==========================================
- Coverage   54.28%   54.27%   -0.02%     
==========================================
  Files         170      170              
  Lines        7346     7346              
==========================================
- Hits         3988     3987       -1     
- Misses       3358     3359       +1     

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[fosterfarrell9]

LGTM.