Add commentmeta tables to an existing MU installation

Needs testing but it’s worked on two existing MU installations so far. I haven’t tested it on a single user setup but I guessed at the code it would probably need… Please share, test, fix, steal, reuse, and recycle this code as needed. There’s probably better ways of doing this, but this got me through it in a pinch.

jjj_addcommentmeta