Handle interpolated transfer legs better

[abhumbla]

As of https://github.com/graphhopper/graphhopper/pull/2519 graphhopper will return actual legs for interpolated transfers. We should do a couple of things

• [x] add a flag to any interpolated leg so that the frontend knows to handle it specially
• [ ] make the mode it uses to transfer configurable separating from connecting mode (e.g. biking to/from transit but walking your bike in a transfer)
• [ ] figure out a way to interpolate transfers for multiple modes at startup
• [ ] not have the router crash by trying to create an interpolated leg down a wrong way street