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
This commit also:
- removes [+ New Topic] behaviour from share, this feature has been duplicated in composer actions, months ago
- introduces our new experimental spacing standard for css: eg: `s(2)`
- introduces a new panel UI for modals
