Handle nested ImageBlocks

[zerolab]

Fixes #849

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.08%. Comparing base (b619a32) to head (4a90395).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
wagtail_localize/segments/extract.py	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #850      +/-   ##
==========================================
- Coverage   93.11%   93.08%   -0.03%     
==========================================
  Files          47       47              
  Lines        4224     4238      +14     
  Branches      547      549       +2     
==========================================
+ Hits         3933     3945      +12     
- Misses        175      176       +1     
- Partials      116      117       +1     

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