Allow commenting on records in the stream

[frankiejarrett]

It would be great to allow conversions to take place on activity via comments, and to send an email alert when "Someone has commented on your activity."

To do:

• Create mockup example of comments in the Stream
• Decide on architecture for storing comments
• Create settings for Comments
• Create email template for comment notifications

[frankiejarrett]

I think this will be another case where the feature is going to be substantial but will only be appreciated by certain types of users, so it should exist as it's own extension plugin.

[frankiejarrett]

A repo for this extension has been created https://github.com/x-team/wp-stream-discussions