Make sure we recalculate user storage on updates

[jayoshih]

When I upload new files, it isn't updating the user storage because the serializer update code doesn't call the File.save() method. This just calculates on the update function

[kollivier]

LGTM!

[codecov]

Codecov Report

Merging #2749 (d58e62b) into develop (68c620b) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #2749      +/-   ##
===========================================
+ Coverage    84.94%   84.98%   +0.04%     
===========================================
  Files          301      305       +4     
  Lines        15227    15265      +38     
===========================================
+ Hits         12934    12973      +39     
+ Misses        2293     2292       -1     

Impacted Files	Coverage Δ
contentcuration/contentcuration/viewsets/file.py	72.22% <100.00%> (+2.52%)	⬆️
contentcuration/contentcuration/models.py	85.49% <0.00%> (-0.24%)	⬇️
contentcuration/contentcuration/tests/testdata.py	93.75% <0.00%> (-0.05%)	⬇️
contentcuration/contentcuration/utils/db_tools.py	69.75% <0.00%> (ø)
contentcuration/contentcuration/views/internal.py	85.18% <0.00%> (ø)
...ntentcuration/contentcuration/db/models/manager.py	93.49% <0.00%> (ø)
...entcuration/migrations/0128_merge_20210112_1828.py	100.00% <0.00%> (ø)
...t_assessment_item_keypair_constraint_migration_.py	100.00% <0.00%> (ø)
.../migrations/0114_assessment_item_unique_keypair.py	100.00% <0.00%> (ø)
...tentcuration/migrations/0127_auto_20210107_1707.py	100.00% <0.00%> (ø)
... and 1 more

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