Take avatar images and process them

Code for accepting user images,and processing them for application usage.

• Accept avatar image import - check is valid image file - check for correct size/proportion -> have a convention for cropping non-square images - convert to png - copy images to class_data/classname/avatars folder
• Take all common image formats.
• Stripping excess whitespace from scanned images.
• Probably automatically convert to PNG by default?
• Resizing to a specific resolution/range of resolutions (probably keep an image, in highest resolution, but possibly keep a copy sized for frequently used size - eg the 10% columns of a 0-100% scaled chart, to save constantly resizing on the fly). Consider that final charts will hopefully be 1920x1080 and projected. #17
• Might need to add whitespace/transparent space to keep aspect ratio but have all avatars sized equally for the purposes of spacing on charts. #17

More of a UI thing: #15, #16

• Would be nice to spawn a 'regular' choose file box with a file explorer, rather than pasting a filepath into the CLI.
• If the file processing might generally hang more than a moment (the time from pressing ok to the time user expects to be able to supply another student's name, possibly spin off separate thread/s for processing, with a "please wait" message if user finishes entering data for the class before the images are processed.