Negative option was leading to a fair amount of confusion, going forward
if we want to allow selection of emails from user selector it must be
supplied with `allowEmails=true`
This corrects a regression in 1f4ace4f which broke invite by emails and
start PM to email
https://stackoverflow.com/a/47822599/17174
Chrome 63 and up start ignoring `autofill="off"`
Per: https://bugs.chromium.org/p/chromium/issues/detail?id=468153#c164
> The tricky part here is that somewhere along the journey of the web autocomplete=off become a default for many form fields, without any real thought being given as to whether or not that was good for users. This doesn't mean there aren't very valid cases where you don't want the browser autofilling data (e.g. on CRM systems), but by and large, we see those as the minority cases. And as a result, we started ignoring autocomplete=off for Chrome Autofill data
So to work around this decision we now explicitly say: autocomplete="discourse"
when we don't want Chrome to randomly fill in addressed (aka. always)
