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.
> Please do not use _ as a prefix for private variables, as it can easily be confused with UnderscoreJS constructs. For example later on, you have a this._times[label] = now() whose syntax is somehow similar to _.times.

Sure. Fixed ;).