Show a message when housekeeping is enabled

If the GSD privacy plugin for automatically trashing files is enabled we should make mention of it in the plug and possibly link to System Settings to configure it.

I have a proposed branch for configuring the settings here: https://github.com/elementary/switchboard-plug-security-privacy/pull/51

The settings URL for this pane is `settings://security/housekeeping`

Review the GSD privacy plugin and the proposed settings work in switchboard-plug-security-privacy PR #51. Use the settings entry point `settings://security/housekeeping` to understand how the pane is opened. Done means the Files plug mentions that automatic trashing is enabled and provides a link to configure it.

Written by the indexing model from the issue text.