in Studio, render compact placeholders for inline discussions.

JIRA: FOR-429

This introduces an alternate rendering of DiscussionModules specifically for Studio.  No overt support for conditional rendering previously existed so a rudimentary mechanism was added.  Test coverage for Studio is provided by this PR; LMS-side coverage is implemented in PR #3183 which is not yet merged as of this moment, i'll rebase before merging this one.

@gwprice @marcotuts 
@dmitchell @auraz @cpennington