This merge improves code of access rights checks when creating or updating
activities. Several points triggers this commit

 * we would like to use _filter_access_rules and override it on activities
   model in order to be more standard;
 * we would like to get rid of sudo used in various CRUD overrides as using
   standard ORM method allows to correctly use current user;
 * we would like to get rid of create_user_id field once we can keep the
   current user as create_uid;

Access rights are a bit updated. They are now the following

 * you can create activities for documents like posting messages (using
   _mail_post_access attribute; if not defined, write access on document
   is required);
 * you can read activities for documents you can read;
 * you can write and unlink activities either following the defined access
   rule (created OR assigned), and otherwise like posting messages (
   using _mail_post_access attribute; if not defined, write access on
   document is required);

In this merge we also hide edit / mark as done / cancel buttons for
activities the current user cannot modify.

Finally create_user_id field related to activities is replaced by create_uid
everywhere.

This commit is linked to task ID 1856417.