Handle autocomplete of private message recipients with comma in names

[neiljp]

Private message validation currently fails with users who have , in their full name.

In addition, autocomplete currently uses , as a separator for multiple recipients.

It may be one or both of these that combine to cause a problem.
However, as it currently stands the recipient box is simply text, so some text separator of recipients would be necessary, which could occur in any full name.
The full solution to this may therefore require a 'pill' based approach for validated users, which would be a much larger project but also expand to a general solution for eg. the message search box.

This issue is not currently a high priority, but could lead to a larger project.