Add comment to "tweak"

[amitshesh]

Is your feature request related to a problem? Please describe.
It is. Currently Pawtograder allows an instructor to override the number of points a student gets with a "tweak". This works, but there is no place to document the reason for the tweak. In the case of a multi-instructor large class, remembering what the tweak was for and who put it there is cumbersome. Often this documentation is needed long after the tweak is actually done.

Describe the solution you'd like
An instructor should be able to add a note to a tweak that shows up along with the tweak.

Describe alternatives you've considered
Asking other instructors about whether they put in a tweak, and what issues were included in the tweak.

Additional context
Add any other context or screenshots about the feature request here.

[jon-bell]

@amitshesh is it ok if the student can see the tweak comment, too?