As a project member, I can leave comments on a project or deliverable

on activity tab, I have a box to leave comments

• box initially reads "Write a comment..."
• can type in a message and hit return for the message to appear on the tab

https://projects.invisionapp.com/d/main#/console/12114617/283302856/preview

AC:

• the placeholder on the box to type a comment is 'Write a comment..."
• when the user hits return, the typed message is sent and saved as a comment