Use render_map to render the admin settings map

This way we remove duplication.

Note that to check whether to render the button to remove a marker,
we're checking whether the map location belongs to a mappable. This
means we're changing the code that renders the map in the "new proposal"
and "new investment" forms so the map location belongs to a proposal or
investment. We're association the map location to a new record because
writing something like:

```
def map_location
  proposal.map_location || MapLocation.new(proposal: proposal)
end
```

Would change the `proposal` object because of the way Rails treats
non-persisted `has_one` associations. Although probably safe in this
case, changing an object when rendering a view could have side effects.

Also note that we're changing the HTML ID of the map element from
`admin-map` to `new_map_location` (the latter is returned by the
`dom_id` method).  We were only using this ID in tests since commit
289426c1c, so changing it doesn't really affect us.

app/components/shared/map_location_component.rb

@@ -18,6 +18,10 @@ class Shared::MapLocationComponent < ApplicationComponent
       form.present?
     end
 
+    def show_remove_marker_button?
+      editable? && map_location.mappable.present?
+    end
+
     def latitude
       map_location.latitude.presence || Setting["map.latitude"]
     end