Change submission period

[rsomonte]

There are two possible ways of doing this:

1. By allowing the user to edit every task's submission period
2. By allowing only to edit the general setting.

The submission period is the time subtracted from 24h that allows the user to submit a task a little bit before than a biological day. Currently is 2h, which means that the user can submit every 22. In some use cases, it may be useful to be more strict.