[Offline base maps] Show available and est. required storage for offline areas

[gino-m]

trafficstars

This will help users understand how much storage downloaded imagery will require, and how much space is left.

For ex:

[gino-m]

@vittorino I believe @rawbzz already did designs for this - could you please paste in and unassign yourself and remove UX needed label?

[kenstershiro]

@gino-m is this essential for GA or can we push to post-GA milestone?

[gino-m]

[@gino-m](https://github.com/gino-m) is this essential for GA or can we push to post-GA milestone?

Nobody has asked for it so I think post-GA is fine.