Studio: Styling Support for Section/Subsection Settings Modal UI

This work adds styling support for section/subsection settings modal UI (needed for https://github.com/edx/edx-platform/pull/4425) and includes the following steps:

* 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
This ```<br />``` element is not needed here for style or semantics. Would you mind removing it?