Improvements to topology graphs

* whole topology graph gains checkbox to use interface names, and dropdown to set minimum length of lines. This helps avoid interface names overlapping with each other.
* device topology view gains dropdown to set minimum length of lines.
* bin/build_topo_graph.pl gains flag "-m <int>" to set minimum length of lines. This can be set in the crontab entries which rebuild graphs (plus the already-existing "-n" flag if you want interface names)

Fixes redmine #1821