Route splitting

[robinlovelace-ate]

This UI feature will allow people to split routes into sections that correspond to lengths that have the same cross section, as described in CAD diagrams or tools like StreetMix: https://streetmix.net

Started in #114

[dabreegster]

• Scissor icon for split tool
• Unit tests
• Move splitting logic to route-snapper
• Automatically update/set the name for split segments. And/or group the split segments together.

[dabreegster]

This could still use a change in implementation and some unit testing, but the feature is working for v2, so calling it done and tracking further work in dabreegster/route_snapper#32