hide content export <hr>s from screen readers

fixes ADMIN-1286

test plan:
- Navigate to a course's Settings page
- Click on Export Course Content
- Move through the page with a screen reader
- Note that each horizontal rule is read to the SR

Change-Id: Ic04b9beba89f4cc9847528cb16189100123d79c8
Reviewed-on: https://gerrit.instructure.com/158913
Tested-by: Jenkins
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Luke Kingsley <lkingsley@instructure.com>
Product-Review: Nathan Rogowski <nathan@instructure.com>

app/views/content_exports/index.html.erb

@@ -36,7 +36,7 @@
     <div class="alert-error error_message"></div>
   </div>
 
-  <hr />
+  <hr aria-hidden="true"/>
 
   <div id="export_files">
     <% @exports.each do |export| %>
@@ -66,7 +66,7 @@
     <p><em><%= t "Content export files cannot be downloaded after %{days} days.", days: ContentExport.expire_days %></em></p>
   <% end %>
 
-  <hr />
+  <hr aria-hidden="true"/>
 
   <div class="form" style="<%= hidden if @current_export_id %>">
     <% if @context.is_a?(Course) %>