Replace raster icons in the footer with vectors

[osbre]

I found the SVG of the "download cloud" icon here and set the gradient to match the old style.

All icons are licensed with either "public domain" or "MIT".

• https://www.svgrepo.com/svg/485703/download1
• https://www.svgrepo.com/svg/512317/github-142
• https://www.svgrepo.com/svg/475689/twitter-color
• https://www.svgrepo.com/svg/512317/github-142

For consistency, I also updated the previously contributed "download" and "arrow-right" icons to be a part of the same SVG symbol collection in HTML.

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[garazdawi]

thanks!