updates: Show the time we last checked for updates

This adds a label with 'Last checked: HH:MM' towards the bottom of the
"has no updates" page.

https://bugzilla.gnome.org/show_bug.cgi?id=725165
Update the saved timestamp when manually refreshing updates

This ensures the "Last checked: HH:MM" label properly updates on
clicking the update refresh button.

https://bugzilla.gnome.org/show_bug.cgi?id=725165
Updated Spanish translation