This feature allows end users to "defer" topics by marking them unread
The functionality is default disabled. This also introduces the new site
setting default_other_enable_defer: to enable this by default on new user
accounts.
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
