TNL-731: Add possibility to search notes.

**Ticket:** [TNL-731](https://openedx.atlassian.net/browse/TNL-731)
**sandbox:** http://tymofij.m.sandbox.edx.org/courses/edX/DemoX/Demo_Course/edxnotes/

@explorerleslie,
@tymofij , @olmar , @jmclaus please review.
> Annotator-store search results is already sorted by updated field: https://github.com/openannotation/annotator-store/blob/master/annotator/elasticsearch.py#L243

Great! Removed redundant sorting.