[FIX] purchase: add purchase_order_count in kanban for purchase users only

No need to set account.group_account_manager for the field supplier_invoice_count, as account.group_account_manager implies account.group_account_user
[FIX] mail: when having only mail installed, do not show any 'share with my followers' compose box. This comes only with hr, for the inbox. This was probably forgotten when updating the mailboxes hr-goal and hr-related gamification / chatter stuff.
[FIX] mail: posting on the user should be redirect to its partner, not nowhere.
[FIX] mass_mailing: fixed label of ab testing not displayed in mailing
form view when not using campaigns; however this option should be displayed.
[CLEAN] mail_message: removed an unused import