Auto-create alias pages after page is completely saved

[kaedroho]

Fixes: #346
Fixes: #391
Also provides a better fix for #245
CC: MozillaFoundation/foundation.mozilla.org#7356 @TheoChevalier

When you tried to publish a page that contains new inline panel items, Django would crash with an integrity error.

This behaviour is caused by:

• Wagtail Localize calls .copy_for_translation to create the alias pages during the post_save signal that was triggered when the source page is published
• At this point, the source page itself is saved but its inline panel items are not
• .copy_for_translation calls .copy_child_relation in django-modelcluster to copy the inline panel objects to the new alias
• .copy_child_relation does not copy the objects, but instead just reassigns them to the new page. This relies on the fact that the page being copied wouldn't be saved again in this request
• After calling .copy_child_relation, the .copy_for_translation then sets all the locale fields of the inline panel objects to the target language. But as they are actually the same Python objects as the source page, this inadvertently updates them on the source page as well
• After the post_save signal is finished, Django then attempts to save the inline panel objects on the source page. This leads to the integrity error because their locale is set to the target language instead of the source, and there is a unique constraint on that field which raises a conflict with the already saved inline panel objects on the alias page.

The workaround to this I have implemented here is to create the alias pages using the after_create_page hook instead of the post_save signal. This is always run after the many to many objects are already saved avoiding the underlying bug in django-modelcluster.

[codecov-commenter]

Codecov Report

Merging #454 (a51f446) into main (1ba776c) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #454      +/-   ##
==========================================
- Coverage   93.10%   93.09%   -0.01%     
==========================================
  Files          35       35              
  Lines        2812     2808       -4     
  Branches      445      442       -3     
==========================================
- Hits         2618     2614       -4     
  Misses         98       98              
  Partials       96       96              

Impacted Files	Coverage Δ
wagtail_localize/synctree.py	93.50% <100.00%> (-0.33%)	⬇️

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