Make rubric prompt display and instructions optional

TIM-410: if users want rich prompts, they can do that with an HTML
Components above the ORA2 question, but doing this makes the prompt
instructions and the prompt box look bad. This adds an optional
'with_instructions' attribute to the prompt tag for toggling the prompt
instructions separately from the prompt. This also adds a check so that
neither the prompt box nor the instructions will display if the prompt
is empty or whitespace-only.

Includes tests for each of these conditions.

@stephensanchez @wedaly