Update conversation icon on user activity sidebar profile to explain messaging restriction when in place

During implementation of private messaging through a proposal author's tooltip profile, we decided to hide the private message link when the author has opted to only receive messages from people they have followed. See more details on the Github issue.

@carol then pointed out that the messaging icon on the user activity sidebar behaved differently, letting a user navigate to the messaging thread before seeing a warning that the messaging is disabled for that participant.

She suggested we add the warning message right on hovering the messaging icon there, saving them frustration from clicking on the link and finding out they couldn't message at all.

I'm adding this as a separate issue to decouple it from the first one, which has a PR ready for merging and already reviewed on a staging app. And so we can discuss potential alternatives as well.