Increase height of message box on invite workflow

Summary:
At least one user wanted to type more text here, and it seems reasonable that administrators may want to write a couple of paragraphs.

I didn't make this short for any particular reason, I just wasn't sure what the workflow would look like as I was building it.

Test Plan: Loaded page, saw normal height text area.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11779