Only add image to answer where the sessions match

[jmsmkn]

As answer image types are editable it could be the case that there are multiple pending upload sessions for an answer at the same time. The value of the latest upload session pk is always stored in the answer. As the upload sessions may be processed out of order (no FIFO guarantees) this PR only allows the image to be assigned to the answer if it originates from the session expected by the answer. Older versions of the answer_image are still saved and tracable through the answer history.

[codecov]

Codecov Report

Merging #2005 (3a2a474) into master (39f753f) will increase coverage by 0.00%.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master    #2005   +/-   ##
=======================================
  Coverage   93.25%   93.25%           
=======================================
  Files         683      683           
  Lines       26006    26008    +2     
=======================================
+ Hits        24252    24254    +2     
  Misses       1754     1754           

Impacted Files	Coverage Δ
app/grandchallenge/reader_studies/tasks.py	97.14% <66.66%> (-2.86%)	⬇️
app/grandchallenge/components/models.py	92.21% <0.00%> (+0.29%)	⬆️

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[miriam-groeneveld]

Looks good to me. Didn't know about casefold()

[jmsmkn]

Yes, this is the way to do caseless string comparison, nasty things can happen otherwise:

https://eng.getwisdom.io/hacking-github-with-unicode-dotless-i/
https://www.djangoproject.com/weblog/2019/dec/18/security-releases/