Selected course is easier to identify

It was not obvious which course a user had selected.

addresses https://github.com/kelvinhsung/GTCS12/issues/27
Readability, returns null on invalid input
Site description is easier to read

Adding a background color and increasing the font size makes the
description easier to read. The description is seving a dual-purpose as
introduction text and needs to be easily read.
Added function to return selected course on a page

Eventually, this function will take care of remembering previously
selected courses.
Refactored course-selector into separate file

The course-selector code was shared among multiple pages and didnt need
to be duplicated. Separating it into a separate file allows it to be
modularly added to pages. The pages still need to handle the
initialization of the selectors state however.

TODO: fix the isOwner bug by adding functionality to check if the course
belongs to the professor