Bring TO: Field into line with taxonomy free-tagging.

Summary

Taxonomy (multiple tags) has many UX improvements being added to it. See Active Tags, Chosen, or Autocomplete Deluxe. Privatemsg doesn't allow these features as it is built slightly differently.

Description

Hi Berdir and other maintainers,

I had a favour to ask for the TO: field in privatemsg_new. It is basically the same as taxonomy, but there have been a couple of features added to differentiate it.

The main difference is privatemsg parses the existing string so that the autocomplete does not include duplicates. Privatemsg enforces the ',' operator on the user string. Cool feature, but is it worth it? If you switch to one of the active tag libraries, you're stuck with the ',' on all your recipients. This functionality is not included in major email systems like GMail and Hotmail (check yourself!). And what are the odds that someone is going to forget who they added in the message string! :)

I'd therefore request to remove the check-duplicate functionality completely and implement the free-tagging widget as is, changing only the autocomplete path. I think you'll make some great gains in UX.