Auto-erase files on Data Providers

[natacha-beck]

It could be useful to have registed files automatically deleted
from some data providers. If the provider is clearly meant only
as an intermediate holding place while files are transfered
from one place to another official data provider, for instance.
On BrainStorm, SFTP-Incoming is such a DP.

Whenever a user has files on such a DP, the userfile index
page should display a persistent warning in red at the top,
something like:

You have 12 files on SFTP scheduled for automatic deletion in 21 hours.

The deletion would be performed by the Portal Janitor (see Issue 1756 || #37 ).

Related: 1756 || #37, 2172