[IMP] maintenance: improve activity management on maintenance request model

This commit improves maintenance requests management through a better
integration of activities and addition of automated activities. Several
things are done in this commit :

 * automatic activities generation is added for maintenance requests.
   Activities are generated to remind assigned users about maintenance to
   perform. They are also automatically removed or updated if the scheduled
   date is changed or if the request is done to avoid bloating users with
   unnecessary activities;
 * a menu to configure activity types is added. Indeed equipment managers
   should be able to see and configure activity types related to their job;

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