Add cron job to clean avatars folder

[Juansecu]

Since there are some scenarios where avatars can be stored in the avatars folder without the user being registered successfully, it is a good idea to clean the avatars folder periodically, so unused avatars are removed automatically.

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.