Automatically clear old tasks from Alyx

[k1o0]

With their logs, the tasks table in the db tasks up a significant amount of space. The DAWG decided the following:

1. Delete tasks after sign-off (perhaps with ibllib helper function)
2. Delete tasks that are set to Complete (after, say, two weeks)
3. Delete old pipeline tasks
4. Delete tasks older than, say, 2 years

The deletion can be done with a cron. One consideration is how to deal with cortexlab alyx.