Use png images in generated reports

[nkelemen18]

• Added generated and optimized .png images (generated from .svgs).
• Report generation refactored to use png images
• Testcases refactored to check the usage of png images
• Old, SVG images removed

Based on #721, fixes #558

[mc1arke]

I don't think we should delete the SVG images, as anyone with existing PR/MR decoration referencing these images will fail to get icons when opening those PR/MRs.

Given the comments on the original issue, have you confirmed these images render properly for you on a system where the SVGs were previously failing to render?

[nkelemen18]

Hello,

Keeping the old, svg images is a good idea, I've restored them.

Yes, I can confirm, that png images are working in GMail. You have to allow to load images once, but after that it renders the images as you can see:

They are a little bit misaligned, but this error comes from GitLab's Markdown renderer: images have 8px bottom margin

The following image cropped from GitLab's UI:

[mc1arke]

Merged, thanks for the contribution!