When user logs out, unload web map

[mikewilburn]

If a user is currently viewing a web map, then elects to log out of the account which gives access to the web map, we should remove that web map from the map.

We could do this by simply restoring the default map view when first starting up the app.

[codergrl]

@mikewilburn this is completed, please verify

[mikewilburn]

Confirmed on iOS that when the user logs out, the currently-loaded web map will disapear from view and the default basemap that opens when the app starts up is loaded instead.

[mikewilburn]

Confirmed on Android that upon signing out, the currently-loaded web map is being wiped from the display and the map is restored to just the basemap the user sees upon first opening the app.

[mikewilburn]

Confirmed this works as expected on UWP. When the user logs out, the currently-displayed web map is removed from the map view.

[mikewilburn]

Not fixed for WPF.

Unlike on UWP where, in order to view a chosen web map, the selection window is removed from view, WPF raises a pane for selecting a web map which does not obscure the map view from view. This allows the selection pane to remain exposed while the user interacts with the map. If the user signs out while this view is still visible, the user is still permitted to view these web maps.