Update the toolbar for the new viewport width controls

Per CHANNELMGR-945, and based on Carolien's designs (to be added), update the Channel Manager toolbar for for the new viewport width controls (See also HSTTWO-3851).

• use the channel-specific widths, if specified
• fallback to the 320px / 720px / 1280px defaults
• add option to render the channel responsively, and make it the default choice
• rearrange toolbar controls per spec (Changes dropdown moves to the left?)
• make sure all fixed widths use a width constraint (rather than min-width) on the iframe