Limit upload image size

[demery]

Investigate limiting the maximum size of user uploaded images. Will likely need to test image size (by pixels? by bytes?) before constraining size.

See instructions here for using Shrine for image sizing:

https://shrinerb.com/docs/processing

See instructions here for adding the image_processing gem:

https://github.com/janko/image_processing

You'll need to add apt-get install libvips to the Dockerfile

Try a size limit of 2000 x 2000 pixels:

magick.resize_to_limit!(2000, 2000)