Add geolocation support to start journeys at any place

[lenaschimmel]

Currently, each journey must start at some stop/station. The monitor can already handle walks and searching / grouping multiple nearby stops. It would be easy to let a journey start at any geolocation.

Possible ways to enter a geolocation:

• Use device location. This needs Java Script and only works on mobile phones (support on desktop browsers is terribly broken), but is relatively easy.
• Let the user enter an address. This needs geocoding, which can be done with e.g. photon.
• Let the user enter a (lat, lon)-coordinate. This would be the easiest, but nerdiest solution. (And least useful for the average user). Anyway, there might be use cases.

(Each of the above should get a separate issue for progress tracking)