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/admin/settings/map_form_component.rb

@@ -1,7 +1,18 @@
 class Admin::Settings::MapFormComponent < ApplicationComponent
   attr_reader :tab
+  use_helpers :render_map
 
   def initialize(tab: nil)
     @tab = tab
   end
+
+  private
+
+    def map_location
+      MapLocation.new(
+        latitude: Setting["map.latitude"],
+        longitude: Setting["map.longitude"],
+        zoom: Setting["map.zoom"]
+      )
+    end
 end