2.4 compat - fixes and enhancements for courses in Awesomebar

Removes unnecessary "Current course" node, replacing it with the child directly.  Also brings back the missing list of courses under "My courses".
Awesomebar improvements and fixes

Including:
* Removal of legacy onclick handlers - this stops JS errors when the Navigation block is not present
* Promotion of the course node under "current course" to direct child of nav menu (removal of "current course" node in between)
* Fix categories in My Courses branch (if enabled)
Awesomebar: add setting to hide "courses" branch from non-logged-in users
Remove compatibility code that's no longer required

Required Moodle version is jumping to 2.3, so some old compatibility code is now unnecessary.
Version bump to 1.7

Also bumps minimum required Moodle version to 2.3.
Re-add 2.4 compat fix accidentally removed in old code prune
Awesomebar: performance improvements

Don't call find_expandable - use existing $isbranch variable to determine if it's a branch.

Also cache activities returned by generate_sections_and_activities.
Awesomebar: Add new setting to toggle inclusion of activity nodes

Defaults to off, which is a change in behaviour from previous versions of Decaf,
but gives a performance boost in most cases.
Version bump to 1.7.1