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.
> or, even better,
> 
> try:
>    url = settings.EDXNOTES_INTERFACE['url']
> except (AttributeError, KeyError):
>   raise ImproperlyConfigured(_("No endpoint was provided for EdxNotes."))

Yes, it's much better. Fixed.