Missing param error when deep linking

If one wishes to deep link to the ILP page, for example from a portal, this causes a missing parameter error. Changing the required_param to optional_param for user_id removes this and the global USER can then be used to return one's own ILP when no user_id is passed.

The code approach used is taken from user profile but one can seemingly get the same effect with the following
$user_id = $PARSER->optional_param('user_id', $USER->id, PARAM_INT);
Merge pull request #2 from jamesballard/master

Missing param error when deep linking