Add roundtrip option

**Description**
* Create design proposal for a "Roundtrip" toggle when adding a trip
* When toggling the "Roundtrip" option, show an additional field for `Return date`
* After submit, create two trips (Ex. Zurich -> Paris 2019-10-18 AND Paris -> Zurich 2019-10-20)

**Specs**
* The map preview should still show the trip