User control of uploaded image thumbnail size

[jegelstaff]

Right now uploaded images get a fixed width max of 200px, this should be user controlled in the settings for the element. Width would then be overridden in the markup somewhere. Default width is in the formulize.css file right now.

Also, user should be able to control whether thumbnails are displayed or not in the first place, rather than it being all images all the time.

[jegelstaff]

Related to issue https://github.com/jegelstaff/formulize/issues/445