Disable comments based on the current request owner

[rebkwok]

Currently anyone can add a comment, at any stage in the request process. With the new turns, we want to limit all editing to only the current owner. The exception is during an author's turn, where we might want to allow a project collaborator who isn't the author to answer output-checkers' questions.

In author's turn:

• author can comment
• any user with access to the workspace can comment

In output-checker's turn:

• output-checker can comment

In final status:

• no-one can comment