Migrate UUIDField token_uuid in Invitation model

[hackdna]

Toward #2592

[codecov]

Codecov Report

Merging #3421 into develop will decrease coverage by 0.09%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           develop    #3421     +/-   ##
==========================================
- Coverage    68.68%   68.59%   -0.1%     
==========================================
  Files          371      371             
  Lines        24366    24276     -90     
  Branches       943      943             
==========================================
- Hits         16735    16651     -84     
+ Misses        7631     7625      -6

Impacted Files	Coverage Δ
refinery/core/test_views.py	100% <100%> (ø)	⬆️
refinery/core/test_serializers.py	100% <100%> (ø)	⬆️
refinery/core/models.py	82.7% <50%> (-0.81%)	⬇️

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