fix negative variable evaluation on formula questions

fixes CNVS-9970
If you use a negative variable as the first element in a formula
expression, formula question operation breaks.  For example, when you
put:

-[x]+5

in the body of the formula question,  and then attempt to put:

-x+5

in the formula definition, the formula question JavaScript does not know
how to parse the expression. This commit adds graceful logic for these cases.

Test plan
- Create a formula question with the initial variable cast as negative
- Formula definitions should be calculated correctly.
Regression
- Confirm that formula questions still work correctly with positive
  variables

Change-Id: I8d2c6f596bb0211e375c5a347dfa8a1ea2870271
Reviewed-on: https://gerrit.instructure.com/33003
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Stanley Stuart <stanley@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Josh Simpson <jsimpson@instructure.com>