fix external svg CORS issue

[corentinbettiol]

Fix #38 (more a temporary workaround than a permanent solution).
Svg files are now embedded in backgroung-image css in order to prevent CORS errors.

The info icon is still blue, thanks to the fill property.

But the other icons won't turn blue if selected.

[corentinbettiol]

(I don't really know why one test is failing, but I think it's because the test package used is broken)

[codecov-commenter]

Codecov Report

Merging #108 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            master      #108    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           68        68            
  Lines         1286      1465   +179     
  Branches        87       123    +36     
==========================================
+ Hits          1286      1465   +179     

Impacted Files	Coverage Δ
...gocms_bootstrap4/contrib/bootstrap4_card/models.py	100.00% <0.00%> (ø)
...gocms_bootstrap4/contrib/bootstrap4_grid/models.py	100.00% <0.00%> (ø)
...gocms_bootstrap4/contrib/bootstrap4_link/models.py	100.00% <0.00%> (ø)
...gocms_bootstrap4/contrib/bootstrap4_tabs/models.py	100.00% <0.00%> (ø)
...ocms_bootstrap4/contrib/bootstrap4_badge/models.py	100.00% <0.00%> (ø)
...cms_bootstrap4/contrib/bootstrap4_alerts/models.py	100.00% <0.00%> (ø)
...ms_bootstrap4/contrib/bootstrap4_content/models.py	100.00% <0.00%> (ø)
...ms_bootstrap4/contrib/bootstrap4_picture/models.py	100.00% <0.00%> (ø)
...s_bootstrap4/contrib/bootstrap4_carousel/models.py	100.00% <0.00%> (ø)
...s_bootstrap4/contrib/bootstrap4_collapse/models.py	100.00% <0.00%> (ø)
... and 3 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1b93b04...ef38e98. Read the comment docs.

[FinalAngel]

thank you @corentinbettiol