[IMP] mail: added message_last_update field. This field is a datetime
field, updated everytime message_post in called. This allows to build filters, search or
actions based on the last message date, for reporting for example.
[MERGE] Sync with trunk
[IMP] mail: message_last_update -> message_last_post + only update when having a subtype
[IMP] project_issue: add last post in groupby
[IMP] crm: add message_last_pos tin groupby for leads and opportunities
[MERGE] [IMP] mail: added message_last_update field. This field is a datetime field, updated everytime message_post in called. This allows to build filters, search or actions based on the last message date, for reporting for example.

Weekly groupbys have been added in project_issue and crm, for leads and opportunities.