DEV: port sidebar new topic component into theme (#129)

This moves the [sidebar new topic button
component](https://github.com/discourse/discourse-sidebar-new-topic-button)
into the theme directly

If you have installed this theme previously, you should manually remove
the component.

scss/sidebar-new-topic-button.scss

@@ -0,0 +1,48 @@
+.navigation-controls {
+  .topic-drafts-menu-trigger,
+  .fk-d-button-tooltip {
+    display: none;
+  }
+}
+
+.sidebar-new-topic-button {
+  flex: 1 1 auto;
+
+  &__wrapper {
+    box-sizing: border-box;
+    display: flex;
+
+    .mobile-view & {
+      margin: 0 0 1rem;
+    }
+
+    &:has(.topic-drafts-menu-trigger) {
+      .sidebar-new-topic-button {
+        border-radius: var(--d-button-border-radius) 0 0
+          var(--d-button-border-radius);
+        border-right: 1px solid var(--primary-300);
+      }
+    }
+
+    .fk-d-button-tooltip {
+      flex: 1 1 auto;
+    }
+
+    .topic-drafts-menu-trigger {
+      flex: 0 1 auto;
+      margin: 0;
+      border-radius: 0 var(--d-button-border-radius)
+        var(--d-button-border-radius) 0;
+    }
+  }
+}
+
+li.sidebar-section-link-wrapper[data-list-item-name="New Topic"] {
+  display: none;
+}
+
+.has-full-page-chat {
+  .sidebar-new-topic-button__wrapper {
+    display: none;
+  }
+}