Add rel="noopener" to target="_blank" links

The noopener construct protects from some nasty clickjacking attacks. We
can apply them savely to all our links since we don't rely on the
previously used page.

Some more details: https://mathiasbynens.github.io/rel-noopener/

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

public/views/shared/refresh-modal.ejs

@@ -14,7 +14,7 @@
                 </div>
                 <div class="new-version" style="display:none;">
                     <h5><%= __('New version available!') %></h5>
-                    <a href="<%- url %>/s/release-notes" target="_blank"><%= __('See releases notes here') %></a>
+                    <a href="<%- url %>/s/release-notes" target="_blank" rel="noopener"><%= __('See releases notes here') %></a>
                     <br>
                     <strong><%= __('Refresh to enjoy new features.') %></strong>
                 </div>