Add first rought support for exercise images
Update requirements
Render exercise thumbnails with easy_thumbnails

Also add delete methods that remove the image files from the filesystem
Send in the REST API the URLs for the thumbnailed pictures
Add flag to indicate an image is the main one for an exercise
Also use the image gallery in the mobile version
Improvements to image galery on mobile version
Show the exercise's main image on exercise list (mobile)
Update changelog
Fix issue with CSS with category in autocompleter
Show the exercise main image in other lists and overviews
Also send the thumbnail's image settings along with the URL
Reset the exercise cache when deleting and saving images
Add a maximal height for the autocompleter result list
Add pop over menu to main exercise image in detail page
Update names of thumbnail sizes
Only explicitly set the encoding of the form when set
Explicitly order exercise images by ID, needed in many databases
Add management command to sync remote exercise images to local DB
Update testcase to handle file uploads, also do a little clean up