Clarify impacts of de-selecting "Allow downloads" under Account settings

[maxkadel]

It is not clear to repository administrators that un-checking the "Allow downloads" box means that viewers (e.g. Universal Viewer) also will not work for unauthenticated users, since those viewers require the ability to download files in order to function.

We should ensure that the tradeoff is clear at the point when repository administrators make this decision.

In order to see the setting, log in as an administrator, go to admin/account/edit, first checkbox, after Analytics Configuration.