Add support for external icon libraries

[arnaudvergnet]

Based on #948. Only the last 2 commits are from this PR.

Adds a new script icon_libraries.py inside of a tools directory. This script allows users to clone an icon library such as https://pictogrammers.com/library/mdi/ inside of a static files directory in the backend. The frontend will then be able to fetch those icons without any change in the schema.

The script takes a single argument telling it which icon library to clone. For now only mdi is supported.

Example usage:

python tools/icon_libraries.py --library mdi

This script can also be invoked from the Makefile to generate all known libraries:

make icon-libraries

The script executes the following steps for MDI:

• The git repository containing the icons is cloned into the /tmp/mdi directory
• Each SVG icon is converted to png and copied to public/media/icon-libraries/mdi using cairosvg
• The file meta.json from the repo is adapted to create the file public/media/icon-libraries/mdi/icons.json, listing all available icons, their possible aliases and tags (to implement a nice search feature in the frontend)
• Once all the icons are copied, the file public/media/icon-libraries/index.json is updated/created to list all available icon libraries, listing their internal id and display name.

This script was designed with adding support for new icon libraries in mind.

The last commit updates the Django schema to add a new source property in the StyleImage model. This property will be used by the frontend to save from which icon library it was originally from.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.91%. Comparing base (f9eda11) to head (417b18f).
⚠️ Report is 38 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #969      +/-   ##
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- Coverage    95.93%   95.91%   -0.02%     
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  Files           85       85              
  Lines         3910     3892      -18     
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- Hits          3751     3733      -18     
  Misses         159      159              

Flag	Coverage Δ
backend-unittests	95.91% <100.00%> (-0.02%)	⬇️

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