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.
@polesye 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```.