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
* move button into login modal with social buttons
* adds email link next to login field when filling it
* adds proper validation messages
* improves forgot password flash clearing
* more tests
This allows users who entered a typo or invalid email address when
signing up an opportunity to fix it and resending the confirmation
email to that address.
