Show inactive sources, to notice sources that have recently stopped working

[SjonHortensius]

I've had a few issues where nodes would no longer send messages to my graylog server, I am familiar with the 'Sources' page and it's the best tool to start detecting these sort of issues; but it would be nice if I could somehow retrieve a list of of Sources and their last message over a certain period (eg a month).

[SjonHortensius]

If javascript/src/components/sources/SourceOverview.jsx could be extended to query sources in the next (bigger) slot as well; and show the diff in a separate 'Inactive sources' group that would fix this.

Alternatively the existing rows could highlight recently-added/recently-removed sources; where the latter obviously show a count of 0 messages