1. commit before pulling David's home page
1. Resolve merge conflict in admin page, now admin = newadmin page
1. Added "POST /Registrations/dropAndGetEnrolledSections" to perform both drop and getEnrolledSections methods;
2. Added security checks for all section mutators in sections_controller;
3. Added "POST /Sections/getSectionsByInstructor"
4. Disable all validations for "day" column but calculate the day field in the BE on creating the section.
   Now to create a section, there is no need to input day field