Improve handling of invalid federated (and regular) shares

As @rullzer mentioned here we should improve the way closed servers with federated shares are handled. The issue in #7001 was fixed with caching and it works, but could be done "in a smarter" way.

I suggest that:

• When a URL for the server of a federated share has been inactive (aka 404) for 30 days, then remove the federated share automatically with cron.php. A solution could be to ping the domain of the receiver once a day for example and after 30 days of "reached time out" remove the share to that specific receiver. Also, when a share is expired - notify the admin when that happens.
• The federated shares are performance checked, so when a federated share performs bad, inform the user with a notification. (As a stage 2 step)
• List all federated share users in the Files app, in addition to "Shared by link". So for example "Federated shares" and list all the users.

I'm sure you guys have even better ideas.

Start by reading the discussion in #7001 and PR #7485, then trace how federated shares are handled by cron.php and displayed in the Files app. The issue proposes several stages—removal after repeated inactivity, admin or user notifications, and listing federated users—so the scope and acceptance criteria need to be clarified before implementation.