Add a global setting to set a mapbox access token

[abey79]

#6561 adds a new map view to rerun. In order to enable mapbox as background, a mapbox token must be provided. Initially, it's done via the RERUN_MAPBOX_ACCESS_TOKEN, but that setting should be editable from the UI and persisted with the rest of the app settings.

[tfoldi]

it is not just a Mapbox token; we can easily add Google Bing, or other map providers - most need tokens for access. (I believe what you eventually need is some sort of centralized secret management.)