misc: improve image compression (#130)
* Compress internship-salaries.jpg with jpegoptim.
Lossless compression with `jpegoptim -sp in.jpg`.
Before: 60,4 KB (61.850 bytes)
After: 59,3 KB (60.824 bytes)
* Losslessly compress favicon.png.
Used optipng, zopflipng and pngout.
Before: 23,0 KB (23.565 bytes)
After: 9,56 KB (9.797 bytes)
* Improve logo.svg compression.
Remove unused IDs and data attributes thus allowing svgo to perform better optimization. Used svgo 1.3.0 after that.
Before: 5339 bytes
After: 4905 bytes
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