Describe the problem and steps to reproduce it:

An imaginary UI flow for commenting might look like this:

• As a reviewer, click the line of a diff to enter a comment
• Type a comment, press save

What happened?

n/a

What did you expect to happen?

The comment should be saved by the backend

Anything else we should know?

I don't see a POST endpoint in the comments API so we need that for starters.

We also need to be able to post all data that will be needed for #6647 , such as file path, line number, and anything else.