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.

spec/components/proposals/form_component_spec.rb

@@ -0,0 +1,17 @@
+require "rails_helper"
+
+describe Proposals::FormComponent do
+  include Rails.application.routes.url_helpers
+
+  before { sign_in(create(:user)) }
+
+  describe "map" do
+    it "renders a button to remove the map marker" do
+      Setting["feature.map"] = true
+
+      render_inline Proposals::FormComponent.new(Proposal.new, url: proposals_path)
+
+      expect(page).to have_button "Remove map marker"
+    end
+  end
+end