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
Some Discourse-supported sharing platforms such as Twitter are unable to determine the end of a unicode URL. If you post a URL "https://example.org/t/测试/1" on Twitter, it will be a link of href="https://example.org/t/"
If the URL contains any unicode character (usually in the slug part) , it must be urlencoded with encodeURI(url) before being passed to source.generateUrl(link, title), or it will be a malformed URL in the sharing tweet.
Despite `navigator.share` being defined the call was failing with this error:
```
sharing DOMException: Internal error: could not connect to Web Share interface.
```
