[Rubrics] Allow Assessor to Digitally Sign Rubric

[AmyAshton]

Problem statement:

Rubrics do not have the option to be digitally signed/authenticated by the assessor - limits use for VET users where evidence requirements require a signature.

Proposed solution:

Add an option to include a signature - would make Canvas a complete LMS for VET users, and would prevent us from supplementing with other programs or resources to meet evidence requirements.

User role(s):

instructor