[Enhancement] Be able to disable mcgill-specific functions

[le-potate]

Description of Problem / Inefficiency:

Since the bot can be used on other servers than the McGill server, the McGill functions aren't always relevant and there should be a way to disable them.

Implementation Suggestion:

@davidlougheed suggested moving mcgill-specific functions to a mcgill cog or to set a flag like @mcgill_specific

[le-potate]

Some functions that aren't relevant to other schools/can only work with the mcgill website:
-?exam
-?course
-?keydates
-?search
-?tepid

Other functions that only work in specific cases:
-?weather can only work for Canada
-metro_status can only work for the Montreal Metro
-cfia_rss can only work for Canada