Add ability for instructor to customize instructions for reflection step

[jleong-openedx]

The currently non-customizable text for Step 2 is:

"These are samples of other student answers for this question. Read them and then compare with your answer below."

Added flexibility for changing this text, and/or perhaps adding text elsewhere for this step could be useful for instructors when they want to provide customized instructions.

[lenglund]

Also mentioned at brown bag session as an ask for the instructions above the rationale box. (The text can be the same for both the initial and final submissions.)