views.py: new function (remove_from_section) to remove a user from a sec...

new function to remove users from sections, using canvas_sdk

section_details.html:
 -- added ids to elements we need to find/manipulate via js
 -- added javascript to:
    call remove_from_section via ajax
    generate success and error messages based on ajax success/failure
    decrement count when user removed from section
    hide name range in header while deleting users
    display 'no users' message when count of users reaches 0
tests.py:
    added tests for remove_from_section

Note: UI design is not complete for deleting users, so x's for deleting users are placeholders and the placement of success/error messages are not to be considered final.