[IMP] fleet: improve activity management on fleet contract model

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

 * fleet vehicle log contract does not inherit from mail.activity.mixin.
   This commit adds the inherit so that fleet users and managers can now
   schedule and manage activities on vehicle contracts. This will help them
   in their daily job;
 * automatic activities generation is added when contracts are nearly expired
   and renewal is required;
 * a menu to configure activity types is added. Indeed fleet managers should
   be able to see and configure activity types related to their job;
 * filters are added to be able to use the systray and to filter the kanban
   view based on activities

Having automated activities allow to replace some messages posted on the
contract. Indeed currently there are messages posted on vehicles about
contracts to renew. As we now have automated activities on contracts to
remind assigned people to renew it the log can be safely removed. It will
lessen noise generated on chatter.

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