Adds new config parameter defaultBaseMapId

[na9da]

What this PR does

Adds a new config parameter defaultBaseMapId which is used as a fallback if no persisted map is found.

Required for TerriaJS/RaPPMap#87

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated CHANGES.md with what I changed.

[zoran995]

This can't be done through init files like in v7? Or this should be third way do define the basemap? There is #4947 to port it.

[na9da]

Ooh! That's right, I forgot that baseMapName existed. I'll close this PR in favor of #4947.