[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/components/share-button-test.js.es6

@@ -1,15 +0,0 @@
-import componentTest from "helpers/component-test";
-moduleForComponent("share-button", { integration: true });
-
-componentTest("share button", {
-  template: '{{share-button url="https://eviltrout.com"}}',
-
-  test(assert) {
-    assert.ok(this.$(`button.share`).length, "it has all the classes");
-
-    assert.ok(
-      this.$(`button[data-share-url="https://eviltrout.com"]`).length,
-      "it has the data attribute for sharing"
-    );
-  }
-});