[Speedgrader] Make the saved comments append to existing text rather than replace

[Seangt]

Problem statement:

When in speedgrader, and leaving overall feedback in the comments area, if you have text typed in the comment box, then select a saved comment, it replaces your existing text rather than appending to it, making you lose everything you typed previously.

Proposed solution:

When selecting a saved comment, simply append that text to the compose area rather than replacing everything.

User role(s):

instructor,ta