Add cron job to freeze accounts

[Juansecu]

Since there are some scenarios where users may not sign in to their accounts for a long time, and the AMF server needs to retrieve offline messages (private messages and friend requests), which can easily increase during the time, it is a good idea to freeze user accounts which have not been accessed during 3 months.

Requirements:

• Schedule the job to 24 hours
• The cron job should be activated by default, but since it applies a heavy-resources task, instance administrators should be able to deactivate it using an environment variable

For further reference about creating cron jobs, check out the Spring documentation.