* Class cm_completion_details builds the custom completion details
for an activity module.

This patch adds some changes to guarantee that OpenBadges 2.1 (OBv2.1)
will return always the same JSON format that OBv2.0 (because the only
difference between both versions it's the BadgeConnect API).

They are thrown in the following circumstances:

 * The dynamic form class doesn't exist;
 * It does exist but it's `check_access` method throws exception

Co-Authored-By: Andrew Nicols <andrew@nicols.co.uk>

This class would belong more appropriately within the 'user' API
(core_user) instead of within the 'core' API, since it is
directly related to user data.

Since the class has only just been added to Moodle, now is a good
time to move it.

This commit adds the missing logic for category events in
get_context() method. Without this fix, users will get errors
if they have category events with userid set to 0.

This reverts commit 9d58d4de, reversing
changes made to 535c0277.