[WIP] FEATURE: merge share and invite actions together (#7021)

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

test/javascripts/fixtures/topic.js.es6

@@ -2136,6 +2136,7 @@ export default {
     pinned: false,
     pinned_at: null,
     details: {
+      can_invite_via_email: true,
       auto_close_at: null,
       auto_close_hours: null,
       auto_close_based_on_last_post: false,