Use Rails methods to get map location input IDs

We were manually generating the IDs in order to pass them as data
attributes in the HTML in a component where we don't have access to the
form which has the inputs.

However, these data attributes only make sense when there's a form
present, so we can pass the form as a parameter and use it to get the
IDs.

We can now define a map as editable when there's an associated form,
which makes sense IMHO.

app/components/proposals/form_component.html.erb

@@ -62,7 +62,6 @@
                    label:    t("proposals.form.map_location"),
                    help:     t("proposals.form.map_location_instructions"),
                    remove_marker_label: t("proposals.form.map_remove_marker"),
-                   parent_class: "proposal",
                    i18n_namespace: "proposals" %>
       </div>
     <% end %>