Make sure field level validation runs when translating snippets

[kaedroho]

Fixes #426

[codecov-commenter]

Codecov Report

Merging #427 (eecbb9a) into main (1d76adf) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #427   +/-   ##
=======================================
  Coverage   93.07%   93.07%           
=======================================
  Files          35       35           
  Lines        2800     2802    +2     
  Branches      445      445           
=======================================
+ Hits         2606     2608    +2     
  Misses         98       98           
  Partials       96       96           

Impacted Files	Coverage Δ
wagtail_localize/models.py	95.85% <100.00%> (+<0.01%)	⬆️
wagtail_localize/test/models.py	97.59% <100.00%> (+0.01%)	⬆️

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