Updates ImageMagick to version 7 and makes its use platform independent

[kalinni]

The code for cropping pictures was tailored to unix systems only, now it should run on all platforms.
Code is also adapted to work with the latest ImageMagick (major changes were introduced with ImageMagick 7, ImageMagick 6 is legacy now).

Details: In the past, "convert" needed to be replaced with "magick convert" to work on windows. Now only "magick" is used on both systems (porting guide: https://imagemagick.org/script/porting.php).

[fkaiserbio]

Thank you for this contribution @kalinni 👍

[mestia]

What about back compatibility with older Imagemagick versions ?
So far imagemagick 7 is not available on Debian/Ubuntu systems, so unless the version bump has important improvements I'd suggest to enable some heuristic to detect the versions of imagemagick on the target system and depending on that run the compatible command.

[kalinni]

The issue is, that with the old implementation the code did not run on windows. ImageMagick 6 used "convert" as its command for what we use it for, which clashes with the native windows command "convert" for file systems. ImageMagick 7 now uses "magick" which avoids the issue.
ImageMagick 6 is legacy and ImageMagick 7 has been around for at least 5 years from what I see - is there really no chance to get it installed for PLIP web? They provide a binary release for Linux here: https://imagemagick.org/script/download.php

As PLIP runs perfectly fine without ImageMagick and we only require the automatic cropping for nice pictures on PLIP web I would not make including back compatibility to ImageMagick 6 a priority unless we can't get it to run for PLIP web right now.