frontend: map: allow setting a custom vehicle image

[ES-Alexander]

Current behaviour

We show default icons on the map for the most common vehicle types Cockpit is used with, but not every vehicle from a given type looks the same, and some users / organisations may wish to use an image that specifically matches their vehicle.

Expected or desired behaviour

Allow a user to provide a custom vehicle image (per vehicle type), which gets used as the icon on the map.

Ideally these would also be supported as images for dynamic points of interest, so that they could be used to visualise other vehicles (e.g. in the same MAVLink network).

Prerequisites

• [x] I have checked to make sure that a similar request has not already been filed or fixed.