[PT 51699153]JavaScript refactor

JavaScript refactor for LetsHire, refer to the story [JS cleanup for LetsHire](https://www.pivotaltracker.com/story/show/51699153). Basically,  it includes:

1. organize JavaScript codes in order to make it more understandable and clear;
2. add JavaScript docs;
3. remove redundant JavaScript codes that is not necessary or not used any more;