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.

spec/system/anon_sidebar_topic_button_spec.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+RSpec.describe "Sidebar New Topic Button", system: true do
+  before { upload_theme }
+
+  it "does not render the sidebar button for anons" do
+    visit("/latest")
+    expect(page).not_to have_css(".sidebar-new-topic-button__wrapper")
+    expect(page).not_to have_css(".sidebar-new-topic-button:not(.disabled)")
+  end
+end