only_office: use a script for default value

This TALES is complex, especially if we need to support python3 as well

bt5/erp5_only_office/SkinTemplateItem/portal_skins/erp5_only_office/OOoDocument_getDataAsBase64DataURL.py

+import base64
+base64_data = base64.b64encode(bytes(context.getData())).decode().replace('\n', '')
+
+return 'data:%s;base64,%s' % (
+  context.getContentType(),
+  base64_data)

bt5/erp5_only_office/SkinTemplateItem/portal_skins/erp5_only_office/OOoDocument_getDataAsBase64DataURL.xml

+<?xml version="1.0"?>
+<ZopeData>
+  <record id="1" aka="AAAAAAAAAAE=">
+    <pickle>
+      <global name="PythonScript" module="Products.PythonScripts.PythonScript"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>_bind_names</string> </key>
+            <value>
+              <object>
+                <klass>
+                  <global name="_reconstructor" module="copy_reg"/>
+                </klass>
+                <tuple>
+                  <global name="NameAssignments" module="Shared.DC.Scripts.Bindings"/>
+                  <global name="object" module="__builtin__"/>
+                  <none/>
+                </tuple>
+                <state>
+                  <dictionary>
+                    <item>
+                        <key> <string>_asgns</string> </key>
+                        <value>
+                          <dictionary>
+                            <item>
+                                <key> <string>name_container</string> </key>
+                                <value> <string>container</string> </value>
+                            </item>
+                            <item>
+                                <key> <string>name_context</string> </key>
+                                <value> <string>context</string> </value>
+                            </item>
+                            <item>
+                                <key> <string>name_m_self</string> </key>
+                                <value> <string>script</string> </value>
+                            </item>
+                            <item>
+                                <key> <string>name_subpath</string> </key>
+                                <value> <string>traverse_subpath</string> </value>
+                            </item>
+                          </dictionary>
+                        </value>
+                    </item>
+                  </dictionary>
+                </state>
+              </object>
+            </value>
+        </item>
+        <item>
+            <key> <string>_params</string> </key>
+            <value> <string></string> </value>
+        </item>
+        <item>
+            <key> <string>id</string> </key>
+            <value> <string>OOoDocument_getDataAsBase64DataURL</string> </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
+</ZopeData>

bt5/erp5_only_office/SkinTemplateItem/portal_skins/erp5_only_office/OOoDocument_viewEditor/my_data.xml

@@ -192,7 +192,7 @@
       <dictionary>
         <item>
             <key> <string>_text</string> </key>
-            <value> <string>python: \'data:%s;base64,%s\' %(here.getContentType(), here.getData().encode("base64").replace(\'\\n\', ""))</string> </value>
+            <value> <string>context/OOoDocument_getDataAsBase64DataURL</string> </value>
         </item>
       </dictionary>
     </pickle>