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.
> Why this check is needed if this tab must be closable?

It's a base class for `RecentActivity` (is not closable) and `SearchResults` (is closable) tabs.
But I'll remove this check to be more informative if somebody will try to close non-closable tab.