FEATURE: Add a shortcut to archive PM

This patch adds a new shortcut to allow archiving private messages. When
on a private message page, just type `a` to archive it. Typing `a` on an
already archived message will move it back to inbox.

spec/system/keyboard_shortcuts_spec.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+RSpec.describe "Keyboard shortcuts", type: :system do
+  describe "<a>" do
+    let(:current_user) { topic.user }
+    let(:topic_page) { PageObjects::Pages::Topic.new }
+
+    before { sign_in(current_user) }
+
+    context "when on a private message page" do
+      fab!(:topic) { Fabricate(:private_message_topic) }
+
+      context "when the message is not archived" do
+        it "archives the message" do
+          topic_page.visit_topic(topic)
+          send_keys("a")
+          expect(page).to have_current_path("/u/#{current_user.username}/messages")
+          expect(UserArchivedMessage.exists?(topic: topic)).to be true
+        end
+      end
+
+      context "when the message is already archived" do
+        before { UserArchivedMessage.create!(topic: topic, user: current_user) }
+
+        it "moves back the message to inbox" do
+          topic_page.visit_topic(topic)
+          send_keys("a")
+          expect(page).to have_current_path("/u/#{current_user.username}/messages")
+          expect(UserArchivedMessage.exists?(topic: topic)).to be false
+        end
+      end
+    end
+
+    context "when on a public topic page" do
+      fab!(:topic) { Fabricate(:topic) }
+
+      it "doesn't archive the topic" do
+        topic_page.visit_topic(topic)
+        send_keys("a")
+        expect(page).to have_current_path("/t/#{topic.slug}/#{topic.id}")
+        expect(UserArchivedMessage.exists?(topic: topic)).to be false
+      end
+    end
+  end
+end