[DocViewer] Allow annotations to 'cross' pages

[fac_crossley]

Problem statement:

When we're commenting on student submissions, its common for a problematic, or exemplary passage/quotation/sentence to begin near the bottom of one page and continue to the next. Right now, we can't highlight such passages. It would be helpful to be able to.

Proposed solution:

modify the annotation tools to allow them to continue from one page to another. We commonly have the need for such annotations; the current limits mean we have to choose which part of a problematic sentence to highlight, which often results in an incomplte, or misleading comment.

User role(s):

instructor

[jpoulos]

I've added this to the Make grading more efficient and effective theme.