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.
make grader type work
A bit more valid css lookup.
Do not hardcode grader types in .underscore template.
Remove JS code related to custom CourseGrader element.
Remove dummy br
Merge pull request #4455 from edx/talbs/style-outline-modal

Studio: Styling Support for Section/Subsection Settings Modal UI