Draft: Adding support for audio files

[Lucas-C]

Implementation for the feature requested in #141

This is currently a draft: the server-side part is OK I think, but it seems like some themes do not support audio files...

Some other things are also currently missing:

• unit tests
• ensure that audio_file.png is properly embedded in the package published on Pypi

Note: the audio_file.png comes from Wikimedia / the Noun project -> https://commons.wikimedia.org/wiki/File:Audio_file_%2861746%29_-_The_Noun_Project.svg

[codecov]

Codecov Report

❌ Patch coverage is 48.71795% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.80%. Comparing base (cf15fcc) to head (56379a1).
⚠️ Report is 84 commits behind head on main.

Files with missing lines	Patch %	Lines
src/sigal/gallery.py	42.10%	11 Missing ⚠️
src/sigal/audio.py	45.45%	6 Missing ⚠️
src/sigal/utils.py	71.42%	2 Missing ⚠️
src/sigal/settings.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #518      +/-   ##
==========================================
- Coverage   88.54%   87.80%   -0.74%     
==========================================
  Files          25       26       +1     
  Lines        2077     2115      +38     
==========================================
+ Hits         1839     1857      +18     
- Misses        238      258      +20     

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