[erp5_web] Add "must-revalidate" caching policy.

This caching policy should be the default for any offline webapp, to prevent any mistake done with the html5 cache manifest.

bt5/erp5_web/PathTemplateItem/portal_categories/caching_policy/must-revalidate.xml

+<?xml version="1.0"?>
+<ZopeData>
+  <record id="1" aka="AAAAAAAAAAE=">
+    <pickle>
+      <global name="Category" module="erp5.portal_type"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>_Add_portal_content_Permission</string> </key>
+            <value>
+              <tuple>
+                <string>Assignor</string>
+                <string>Manager</string>
+              </tuple>
+            </value>
+        </item>
+        <item>
+            <key> <string>_Add_portal_folders_Permission</string> </key>
+            <value>
+              <tuple>
+                <string>Assignor</string>
+                <string>Manager</string>
+              </tuple>
+            </value>
+        </item>
+        <item>
+            <key> <string>_Copy_or_Move_Permission</string> </key>
+            <value>
+              <tuple>
+                <string>Assignor</string>
+                <string>Manager</string>
+              </tuple>
+            </value>
+        </item>
+        <item>
+            <key> <string>_Delete_objects_Permission</string> </key>
+            <value>
+              <tuple>
+                <string>Assignor</string>
+                <string>Manager</string>
+              </tuple>
+            </value>
+        </item>
+        <item>
+            <key> <string>_Modify_portal_content_Permission</string> </key>
+            <value>
+              <tuple>
+                <string>Assignee</string>
+                <string>Assignor</string>
+                <string>Manager</string>
+                <string>Owner</string>
+              </tuple>
+            </value>
+        </item>
+        <item>
+            <key> <string>__translation_dict</string> </key>
+            <value>
+              <dictionary/>
+            </value>
+        </item>
+        <item>
+            <key> <string>categories</string> </key>
+            <value>
+              <tuple>
+                <string>caching_policy/must-revalidate</string>
+              </tuple>
+            </value>
+        </item>
+        <item>
+            <key> <string>description</string> </key>
+            <value>
+              <none/>
+            </value>
+        </item>
+        <item>
+            <key> <string>id</string> </key>
+            <value> <string>must-revalidate</string> </value>
+        </item>
+        <item>
+            <key> <string>portal_type</string> </key>
+            <value> <string>Category</string> </value>
+        </item>
+        <item>
+            <key> <string>title</string> </key>
+            <value> <string>Must Revalidate</string> </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
+</ZopeData>