Final deprecation of core_grades_external::get_grades along with unit tests and related private methods.

The previous method of hardcoded filepaths, specifically for events
belonging to core, meant that any events belongs to core subsystems
were omitted (core_customfield, core_h5p, core_payment, etc).

Match on the component frankenstyle name rather than it's lang
string.

* The helplink has changed so the language customisation check had to be
  updated to use a different selector.
* The totop string has been moved from boost to classic only and the
  string must be moved too.
* The new footer toggle may sit over the [next page] button for the
  timeline block for some browsers (Linux only). Changing the browser
  size to large resolves this.

AMOS BEGIN
 CPY [totop,boost],[totop,classic]
AMOS END

AMOS BEGIN
 CPY [clearsearch,tool_moodlenet],[clearsearch,block_timeline]
AMOS END

If defaulthomepage is set to Dashboard then Site home link
in the primary navigation should point to "/?redirect=0"

This implementation will introduce a qbank plugin
"comment" which will allow users with the capability
to comment in a question. It also implements a
callback for the question preview page to view
and add comments from the preview page. Comment
plugin is implemented using the existing comments
API from the core.

Co-Authored-By: Guillermo Gomez Arias <guillermogomez@catalyst-au.net>
Co-Authored-By: Safat Shahin <safatshahin@catalyst-au.net>
Co-Authored-By: Matt Porritt <mattp@catalyst-au.net>