Add a "back" link to the navigation

This solves half of #271 by implementing a link to take the user back to
the main application. It's assumed that `root_url` without a namespace
returns the main application's root.

app/views/administrate/application/_navigation.html.erb

@@ -8,6 +8,8 @@ as defined by the routes in the `admin/` namespace
 %>
 
 <nav class="navigation" role="navigation">
+  <%= link_to "Back to main application", root_url, class: "navigation__link" %> 
+
   <% Administrate::Namespace.new(namespace).resources.each do |resource| %>
     <%= link_to(
       display_resource_name(resource),

spec/features/navigation_spec.rb

@@ -1,6 +1,13 @@
 require "rails_helper"
 
 describe "navigation" do
+  it "has the link of back to application" do
+    visit admin_customers_path
+
+    navigation = find(".navigation")
+    expect(navigation).to have_link("Back to main application")
+  end
+
   it "highlights the link to the current page's resource type" do
     visit admin_customers_path