Use mosaico graphics service to resize thumbnail.

[agileware-fj]

Fix for Error: Class 'Imagick' not found when uploading images with only GD extension, per #312

[totten]

Agree this code looks pretty sensible. Tested locally with a couple different drivers active, and it seemed to work in each case. There are multiple reports of people using it successfully in #312.

Thank you @agileware-fj @francescbassas !

[JKingsnorth]

I think this patch warrants a new minor release, as without it you still need ImageMagick installed alongside GD to make the extension work @totten / @mattwire

Thanks for the fix!

[mattwire]

@JKingsnorth I'd like to get #371 merged if possible before doing another release - be great if you were able to test?