Add a means to disable the default translation mode

[zerolab]

Fixes #447

This PR adds a new global WAGTAIL_LOCALIZE_DEFAULT_TRANSLATION_MODE setting that allows a developer to disable the default translation editor by setting this value to 'simple'

It also checks for a localize_default_translation_mode attribute on the source model allowing for more specificity (e.g. globally disabled translation mode, but enabled for a particular page type. Or the other way around)

[codecov-commenter]

Codecov Report

Merging #473 (49d8e93) into main (b7a321b) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #473      +/-   ##
==========================================
+ Coverage   93.07%   93.09%   +0.01%     
==========================================
  Files          35       35              
  Lines        2844     2852       +8     
  Branches      447      448       +1     
==========================================
+ Hits         2647     2655       +8     
  Misses        101      101              
  Partials       96       96              

Impacted Files	Coverage Δ
wagtail_localize/test/models.py	97.66% <100.00%> (+0.05%)	⬆️
wagtail_localize/views/submit_translations.py	93.38% <100.00%> (+0.20%)	⬆️
wagtail_localize/wagtail_hooks.py	88.33% <100.00%> (ø)

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