change wording from main naviagtion to global

fixes ADMIN-307

Test Plan:
- Navigate to the main navigation
- Notice it now says global

Change-Id: Idc49a5ddd33414b8ac9296138f010ea726a252cd
Reviewed-on: https://gerrit.instructure.com/130805
Tested-by: Jenkins
Reviewed-by: Ed Schiebel <eschiebel@instructure.com>
QA-Review: Dan Sasaki <dsasaki@instructure.com>
Product-Review: Steven Burnett <sburnett@instructure.com>

app/views/shared/_new_nav_header.html.erb

@@ -19,7 +19,7 @@
   <% js_bundle :navigation_header %>
   <header id="header" class="ic-app-header no-print <%= 'no-user' unless @current_user %>">
     <a href="#content" id="skip_navigation_link"><%= t 'links.skip_to_content', "Skip To Content" %></a>
-      <div role="region" class="ic-app-header__main-navigation" aria-label="<%= t('Main Navigation') %>">
+      <div role="region" class="ic-app-header__main-navigation" aria-label="<%= t('Global Navigation') %>">
       <% unless @current_user.try(:prefers_high_contrast?) %>
         <div class="ic-app-header__logomark-container">
           <a href="<%= dashboard_url %>" class="ic-app-header__logomark">

spec/selenium/test_setup/common_helper_methods/login_and_session_methods.rb

@@ -116,14 +116,14 @@ module LoginAndSessionMethods
   end
 
   def displayed_username
-    f('[aria-label="Main Navigation"] a[href="/profile"]').click
+    f('[aria-label="Global Navigation"] a[href="/profile"]').click
     f('#global_nav_profile_display_name').text
   end
 
 
   def expect_logout_link_present
     logout_element = begin
-      f('[aria-label="Main Navigation"] a[href="/profile"]').click
+      f('[aria-label="Global Navigation"] a[href="/profile"]').click
       fj('form[action="/logout"] button:contains("Logout")')
     end
     expect(logout_element).to be_present