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

spec/system/sidebar_topic_button_spec.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+RSpec.describe "Sidebar New Topic Button", system: true do
+  before { upload_theme }
+  fab!(:group)
+  fab!(:user) { Fabricate(:user, trust_level: 3, groups: [group]) }
+  fab!(:category)
+  fab!(:private_category) do
+    c = Fabricate(:category_with_definition)
+    c.set_permissions(group => :readonly)
+    c.save
+    c
+  end
+
+  before { sign_in(user) }
+
+  it "renders the new topic button in the sidebar" do
+    visit("/latest")
+    expect(page).to have_css(".sidebar-new-topic-button__wrapper")
+    expect(page).to have_css(".sidebar-new-topic-button:not(.disabled)")
+  end
+
+  it "opens the composer when clicked" do
+    visit("/")
+    find(".sidebar-new-topic-button").click
+    expect(page).to have_css("#reply-title")
+  end
+
+  it "shows draft menu when drafts exist" do
+    Draft.create!(user: user, draft_key: "topic_1", data: {})
+
+    visit("/")
+    expect(page).to have_css(".sidebar-new-topic-button__wrapper .topic-drafts-menu-trigger")
+  end
+
+  it "disables button when visiting read-only category" do
+    visit("/c/#{private_category.slug}/#{private_category.id}")
+
+    expect(page).to have_css(".sidebar-new-topic-button[disabled]")
+
+    visit("/c/#{category.slug}/#{category.id}")
+
+    expect(page).not_to have_css(".sidebar-new-topic-button[disabled]")
+  end
+end