Modified checklist extension to use style.less

Modified the extension so that it uses style.less instead of css file.

Testing Done:
Manual testing on Chrome and Firefox:
* Added items to the list
* Checked items off the checklist
* Unchecked items on the checklist
* Deleted items from the checklsit
* Minimize the checklist
* Maximize the checklist
* Delete the whole checklist
* Start a new checklist by refreshing the page

Reviewed at https://reviews.reviewboard.org/r/5932/