Add ?overlays request parameter to control overlays
Add ?overlays request parameter to control overlays

Behaviour on refresh and for hidden overlays is similar as for `?map`...
... except there is a preexisting issue right now where enabled overlays are not shown on refresh, but they are in the shared URL.

Examples:
- Multi: http://localhost:8000/?map=OpenStreetMap&overlays=Hills,Cycling
- Hidden: http://localhost:8000/?map=OpenStreetMap&overlays=FR%20IGN%20Parcelles

Part of #26

opoto/wtracks,opoto/wtracks