block_course_list option to be hidden to guests

I've created a patch which adds a new option to allow the course list to be hidden to guests. The default behaviour of it being shown is kept, but this adds the ability to hide it to guests.

I am using isguest() to test for a guest - not sure if I should be using something different with the new roles system. I basically have done the same as is done for $CFG->block_course_list_adminview - and I cleared up the indenting on config_global.html as it was difficult to see what the select code was doing.

This adds a really useful option so that students get their 'my courses' when they login, but don't show guests all the courses available.