* Implement a ViewedEvent handling system which calls handlers when a
block has been viewed for 5 seconds (configurable).
* Hook up Verticals to register their children blocks with this event,
and submit completions once seen.
OSPR-2093
OC-3358
* Vertical marks blocks completed when viewed.
* Functionality is hidden behind a waffle switch
* Submissions from front-end are limited to known-incomplete blocks
* Upgrades xblock to version 1.1.1
* Related future requirements listed in TODO tagged with EDUCATOR-1778
and relevant opencraft OC-* ticket IDs.
OC-3088