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/controllers/admin/settings_controller.rb

@@ -13,9 +13,9 @@ class Admin::SettingsController < Admin::BaseController
   end
 
   def update_map
-    Setting["map.latitude"] = params[:latitude].to_f
-    Setting["map.longitude"] = params[:longitude].to_f
-    Setting["map.zoom"] = params[:zoom].to_i
+    Setting["map.latitude"] = params[:map_location][:latitude].to_f
+    Setting["map.longitude"] = params[:map_location][:longitude].to_f
+    Setting["map.zoom"] = params[:map_location][:zoom].to_i
     redirect_to request_referer, notice: t("admin.settings.index.map.flash.update")
   end