Improve assignment PDF annotation

Although PDF annotation is quite useful it is still a bit clunky (especially compared to some similar commercial offerings, e.g., Turnitin's Grademark) and this is making uptake slow.

Some common feedback that we have received....

• Comment boxes are too large and obscure text underneath them
• There is no way of selecting text and attaching a comment to it
• There is no way of correcting text (e.g. adding a missing apostrophe). Edit: Please see MDL-55240 as this should address the editing situation.
• There is a lot of "white space" in the annotation window that is not annotatable (can't put comment boxes right to the edge of the screen)