Polishing Style Changes, and Resize Responsiveness

-- Made modal fully (and cleanly) responsive to resizing (tabs, buttons, fields, window), [600px , very large)
-- Changed Default Rubric Feedback Prompt to not include space beforehand
-- Made "add criterion" button fall in line with standards
-- Standardized indentations for options, and their sub elements. Reads like bullet-points by 5px.
A very good question! When trying to optimize the horizontal responsiveness, I realized there has to be a threshold which our features cannot be pushed beyond, and where we instead default to scrolling over them (which is what happens at 600 px). A couple of other modals in studio have a min-width for the same purpose (as does our student facing xblock, among other things).  I chose 600px because it is the smallest size where we can still get tabs on the page, and have fields be legible. Going even fifty pixels less distorts almost all of our fields, and makes the UI pretty unreasonable, both to use and maintain.