patches: add a compatibility patch for `zope_quick_start`

`zope_quick_start` was a dtml that was removed in Zope commit c8e4cd5ca
(Simplify control panel objects and actually make them non-persistent.,
2016-08-14), but it is referenced in /index_html page template, which
contains:

    <tal:text tal:replace="structure context/zope_quick_start" />

As a result, rendering / is an error page because of KeyError: 'zope_quick_start'

In SlapOS setup, we are using / in an haproxy health check, so the
health check fail and zope is marked as down and haproxy does not send
requests to backend.

We could have an upgrader constraint to repair this page template, but
because of this, zope is unreachable through haproxy, so it's easier to
ressort to monkey patching.

product/ERP5Type/patches/OFSFolder.py

@@ -60,8 +60,17 @@ def Folder_isERP5SitePresent(self):
 
 Folder.isERP5SitePresent = Folder_isERP5SitePresent
 
+def Folder_zope_quick_start(self):
+  """Compatibility for old `zope_quick_start` that is referenced in
+  /index_html (at the root)
+  """
+  return 'OK'
+
+Folder.zope_quick_start = zope_quick_start
+
 security = ClassSecurityInfo()
 security.declareProtected(Permissions.ManagePortal, 'isERP5SitePresent')
+security.declarePublic('zope_quick_start')
 Folder.security = security
 InitializeClass(Folder)