Remove Pillow from the backend

Codecov Report

Merging #652 into master will increase coverage by 0.06%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #652      +/-   ##
==========================================
+ Coverage    86.2%   86.27%   +0.06%     
==========================================
  Files         181      181              
  Lines       13341    13307      -34     
==========================================
- Hits        11501    11480      -21     
+ Misses       1840     1827      -13

Impacted Files	Coverage Δ
arkindex/images/models.py	60.32% <ø> (+3.11%)
arkindex/images/tests/test_image.py	100% <ø> (ø)

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merged

mentioned in commit 888ff252

mentioned in issue #476 (closed)