#15 — Roads tab: fire road saved as MultiLineString

[bobinush]

The final public map renders fire roads better when the geometry is
MultiLineString rather than LineString.

Acceptance criteria:

• When saving a fire road whose drawn geometry is a LineString, it is automatically converted to MultiLineString (single-element coordinates array) before the API call
• Walking paths are saved as LineString
• Existing MultiLineString fire roads loaded from the API are rendered correctly without modification