R1:2e8990e9e09b

Store metadata with Differential and Maniphest comments, and store added
reviewers as metadata

Summary:
The "Add reviewer" implementation is super lazy right now since I didn't want to
do a schema change. Man up and add a column. I also plan to store "via"
information here (e.g., via email or via mobile).

NOTE: This schema change may take a while since the comment table is pretty big
in Facebook's install.

This needs a little CSS work but I think it's reasonable for now.

Test Plan:
Made comments on…