erp5_interface_post: add source base category in Internet Message Post.

that is to store the interface from which the message is imported.

bt5/erp5_interface_post/PortalTypeBaseCategoryTemplateItem/base_category_list.xml

 <base_category_list>
+ <portal_type id="Internet Message Post">
+  <item>source</item>
+ </portal_type>
  <portal_type id="Internet Message Post Module">
   <item>business_application</item>
  </portal_type>

bt5/erp5_interface_post/bt/template_portal_type_base_category_list

 Internet Message Post Module | business_application
+Internet Message Post | source
 Letter Post Module | business_application
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