move link management plugin to its own directory

Also add the JavaScript file it uses.
linkmgmt: do not require() extra RackTables files

This does not work with a symlink-based install and a proper way to do
that does not still exist.

getProximateRacks() will be available without inc/popup.php starting
with RackTables 0.20.9; renderCell() will not but there is now a local
substitute.