MDL-41910 user: Allowing LASTACCESS_UPDATE_SECS overwrite
Merge branch 'MDL-41910_master' of git://github.com/dmonllao/moodle
MDL-41845 Activity Completion: Make default selection an option.

Makes an option to either default to the activity default, or default
to COMPLETION_TRACKING_NONE. This leaves open the possibility of
allowing activities to pick an actual default, instead of just on/off.