zoom limits so network doesn't easily disappear if you accidentally scroll too much

[gbader]

Description of new feature

We should have zoom limits on the network view so it is not easy for the user to accidentally make the network disappear by scrolling. We have implemented this in many other network viewer apps. Max mentions that the cytoscape.js scale factor needs to be set appropriately on a per app or network basis - it would be nice if we could figure out a generic method for setting this scale factor e.g. based on the number of pixels on the screen that are used up for the network.