Change the course viewer's style. Add affix to the table header.
Checkpoint for backup.
Fix the enroll button bug.
Bug Description:
When the course detail show, clicking the enroll button will hide the details unexpectedly.
Fix the course selection process.
Add footer function but it's not applied now.
Refacter some code about file structure and adding courses.

Move almost all embeded javascirpt code from course_picker.php to CourseSelecitonCore.js.
Using CoursesHandler object instead of global variable added_courses_list and remove some messy manipulation.
Change the method to store accepted courses and credits info.
Adjusted filter menu generating method.

- Decreasing the usage of global variable.
- Auto-generating the category of insititutes by their ID. For more details, consult core/search_filter_data.php
- Adding object CategoryHandler to deal with the courses' categories
Delete the useless files.
Adjust the filter menu style
Eliminated almost all global variable in CourseSelectionCore.
Change the semester number.
Change the semester number
Delete something
adding .gitignore, removing unnecessary files
Merge branch 'affixTester'