Update mechanism of updating last_activity_datetime.

Earlier:
- last_activity_datetime is updated always in any controller. Even if user did not save his first_name/last_name.

Now:
- last_activity_datetime is updated only in front-end controllers if user saved his first_name/last_name.

Updated Controller and FrontEndController.
Updated migration <init_users>:
- change last_activity_datetime type.