Adds documentation about the system header and footer

doc/customization/system_header_and_footer_messages.md

+# Adding a system message to every page
+
+Navigate to the **Admin** area and go to the **Appearance** page.
+
+Under **System header and footer** insert your header message and/or footer message.
+Both background and font color of the header and footer are customizable.
+
+![appearance](system_header_and_footer_messages/appearance.png)
+
+After saving, all GitLab pages will contain the custom system header and/or footer messages:
+
+![custom_header_footer](system_header_and_footer_messages/custom_header_footer.png)
+
+The GitLab sign in page will also show the header and the footer messages:
+
+![sign_up_custom_header_and_footer](system_header_and_footer_messages/sign_up_custom_header_and_footer.png)

ee/spec/helpers/ee/appearances_helper_spec.rb

@@ -3,38 +3,34 @@ require 'spec_helper'
 describe AppearancesHelper do
   describe '#header_message' do
     it 'returns nil when header message field is not set' do
-      appearance = build(:appearance)
+      appearance = create(:appearance)
 
-      expect(helper.header_message(appearance)).to be_nil
+      expect(helper.header_message).to be_nil
     end
 
     context 'when header message is set' do
-      let(:appearance) { build(:appearance, header_message: message) }
-
       it 'includes current message' do
         message = "Foo bar"
-        appearance = build(:appearance, header_message: message)
+        appearance = create(:appearance, header_message: message)
 
-        expect(helper.header_message(appearance)).to include(message)
+        expect(helper.header_message).to include(message)
       end
     end
   end
 
   describe '#footer_message' do
     it 'returns nil when footer message field is not set' do
-      appearance = build(:appearance)
+      appearance = create(:appearance)
 
-      expect(helper.footer_message(appearance)).to be_nil
+      expect(helper.footer_message).to be_nil
     end
 
     context 'when footer message is set' do
-      let(:appearance) { build(:appearance, header_message: message) }
-
       it 'includes current message' do
         message = "Foo bar"
-        appearance = build(:appearance, footer_message: message)
+        appearance = create(:appearance, footer_message: message)
 
-        expect(helper.footer_message(appearance)).to include(message)
+        expect(helper.footer_message).to include(message)
       end
     end
   end