Implement Pagination for Content libraries

Jira Issue: SOL-4

Provides pagination for content Libraries in the Studio Editing View.

* Partner information: 3rd party-hosted open edX instance, for an edX solutions client (so will need to get some sort of solution merged in)
* Merge Deadline: Next Week, 11th of December
cpennington mentioned that `self.runtime.render_template` is a leftover from XModule architecture, so XBlocks should be doing their own templating. It would be nice to try that, but if `render_template` does more than just rendering templates (e.g. injecting context parameters, wrapping result into something, etc.) it could be quite troublesome.

So I would say it might make sense trying, but if it does not work after an hour or so - abandon this idea :)