Show vehicle properties in popup

[roata]

When a vehicle marker is clicked, a popup window should be shown, with the following information taken from web-ui/src/data/vehicles.csv:

• route number, e.g. 30
• board number, e.g. 3898
• whether it has accessibility features, by displaying a corresponding icon

Notes:

• We can take a Unicode approach, 'WHEELCHAIR SYMBOL' (U+267F), ♿. However, it may look different on different devices, so perhaps it might be a better idea to use a specific icon.
• The route number is in the popup for the case when the user has selected multiple routes to be displayed at the same time (at the moment we don't have this capability, but let's imagine we have it).

Closes #64

[ralienpp]

Draft implementation of this in #64