Add a general comment to a version diff

[kumar303]

User story:

As a code reviewer, I want to be able to enter custom comments for specific code lines in the file and diff viewer.

When looking at a version diff, the reviewer can enter a general comment. This is about adding a comment to the entire entire version diff, not a specific file diff (see https://github.com/mozilla/addons-code-manager/issues/989, https://github.com/mozilla/addons-code-manager/issues/992)

The suggested UI flow per meeting notes is roughly this:

• click on a button at the top to begin entering a comment
• when presented with a list of canned responses, click other
• begin entering text
• press save

┆Issue is synchronized with this Jira Task