Include opacity to colors to allow images in cover blocks not to be tinted

[fblupi]

Fixes #34

If you define 0 opacity, the image won't be tinted.

Example of configuration:

Result:

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.57% 🎉

Comparison is base (110292c) 86.54% compared to head (80ec2f4) 87.12%.

❗ Current head 80ec2f4 differs from pull request most recent head decbfa3. Consider uploading reports for the commit decbfa3 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
+ Coverage   86.54%   87.12%   +0.57%     
==========================================
  Files          32       33       +1     
  Lines         513      536      +23     
==========================================
+ Hits          444      467      +23     
  Misses         69       69              

Files Changed	Coverage Δ
...im/alternative_landing/content_blocks/base_cell.rb	97.50% <100.00%> (+0.83%)	⬆️
lib/decidim/alternative_landing.rb	100.00% <100.00%> (ø)
...ve_landing/content_blocks/content_blocks_shared.rb	90.12% <100.00%> (+0.51%)	⬆️
...b/decidim/alternative_landing/default_opacities.rb	100.00% <100.00%> (ø)

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[davidbeig]

Really like your solution!

Good work!

I have re-run the failing test, since it had nothing to do with this PR, and now we can merge it.