Do ot display Delete Release date for Sections.
Uncomment check for permissions.
Switch modal title according to xblockInfo's category.
Studio: adding styling support for section/subsection settings modal UI:
* revising mark up/semantics for modal form UI
* refactoring and revising styling for modal form UI
* adding basic modal content support styling
* changing section/subsection modal window size to 'large'
* converting subsection grading type selection UI to native select element
Bring back .remove-date for button that removes due date on subsection.