Handle configurable comments relation (Wagtail 2.15 fix)

[gasman]

The 'comments' relation on Page will be renamed to 'wagtail_admin_comments' in Wagtail 2.15, as per wagtail/wagtail#7591, and 2.13.5 / 2.14.2 will allow developers to 'opt in' to the new name to prevent conflicts with third-party packages that also use the name 'comments' (wagtail/wagtail#7600 / wagtail/wagtail#7601). We therefore need to recognise the relation name in use, rather than hard-coding 'comments'.

[codecov-commenter]

Codecov Report

Merging #468 (ad5f385) into main (8ee52c8) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #468   +/-   ##
=======================================
  Coverage   93.13%   93.14%           
=======================================
  Files          35       35           
  Lines        2840     2844    +4     
  Branches      447      447           
=======================================
+ Hits         2645     2649    +4     
  Misses         99       99           
  Partials       96       96           

Impacted Files	Coverage Δ
wagtail_localize/fields.py	92.72% <100.00%> (+0.27%)	⬆️

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[kaedroho]

Thanks for fixing!