Bump a public neighborhood to the top of the Nexus list when someone links to it.

This is a proof-of-concept implementation of the scheme I proposed in [*4 pinned hoods in nexus* on the MOUL forum](http://mystonline.com/forums/viewtopic.php?t=27639&p=414181#p414181). It achieves that public neighborhoods that are in active use stay (near the top) on the list in the Nexus, even after their owners disappear (as is the case for the numerous dedicated Delin & Tsogal hoods that have been made over time), with no manual toggling needed, and no need for pinned hoods.

Tested on MOSS and MOULa. Not tested on Dirtsand.

Requires H-uru/Plasma#461.