Source link display no longer connected to target.quote

In the standalone page template the source links were inside of a span tag with a ng-show="target.quote", so for image annotations they were hidden

Fixes #855
Introducing imagecropper service

For image annotations we have to cut out the selection from an image and display only that, with an optional scaling. Introducing the imagecropper service which handles all this by giving it the image source, the shapeSelector and the container element to put the canvas into.