Support for shortcode-like image feature

[matheusgomes28]

Easily add markdown images with shortcode-like feature. This is mostly a POC for further plugin-like features to allow user shortcodes to be provided

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 89 lines in your changes are missing coverage. Please review.

Project coverage is 4.93%. Comparing base (86f0ef8) to head (4937610).
Report is 1 commits behind head on main.

❗ Current head 4937610 differs from pull request most recent head f61d4f8. Consider uploading reports for the commit f61d4f8 to get more accurate results

Files	Patch %	Lines
admin-app/post.go	0.00%	67 Missing ⚠️
cmd/urchin-admin/main.go	0.00%	19 Missing ⚠️
admin-app/app.go	0.00%	2 Missing ⚠️
app/app.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #42       +/-   ##
==========================================
- Coverage   43.42%   4.93%   -38.49%     
==========================================
  Files          16      13        -3     
  Lines         760     709       -51     
==========================================
- Hits          330      35      -295     
- Misses        386     673      +287     
+ Partials       44       1       -43     

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