Add a task that routinely deletes old messages

[yigitguler]

My thoughts:

• This task should be on by default.
• Delete threshold should be adjjustable in the settings file. The default threshold should be a year. (Any thougts on this?)
• Task should run every day.

[alicertel]

Sounds good to me.

I think easiest implementation would be to prepare the task and let the module user to define celery beat schedule for it.

[yigitguler]

@hacernur Over to you.