Added new module_enabled() function to misc, which, due to caching, will make it faster to see if a module
is enabled or not, rather than using function_exists(). Updated various files to use this new function instead
of function_exists()

Added new setting to schools where courses from the "default" school can be used in non-default degrees and groups.

Added create table instructions to enagagements.install for required tables.

Modified the fp_join_assoc() and explode functions to use a separator of _S- instead of just - (since we might be sending negative numbers).

Removed bug where "Substitute" tab was showing in popup even on "courses added".

Removed "Required by" text/line if we are in the Courses Added block, or if we are looking at a degree with only one "degree" (not combined).

Fixed bug with student_files which was preventing file uploads.