Use a switch to toggle proposal selection

The button to select/deselect a proposal wasn't very intuitive; for
example, it wasn't obvious that pressing a button saying "selected"
would deselect the proposal.

So we're using a switch control, like we do to enable/disable features
since commit fabe97e50.

spec/components/admin/proposals/toggle_selection_component_spec.rb

@@ -0,0 +1,21 @@
+require "rails_helper"
+
+describe Admin::Proposals::ToggleSelectionComponent, :admin do
+  describe "aria-pressed attribute" do
+    it "is true for selected proposals" do
+      proposal = create(:proposal, :selected)
+
+      render_inline Admin::Proposals::ToggleSelectionComponent.new(proposal)
+
+      expect(page).to have_css "button[aria-pressed='true']"
+    end
+
+    it "is false for not selected proposals" do
+      proposal = create(:proposal)
+
+      render_inline Admin::Proposals::ToggleSelectionComponent.new(proposal)
+
+      expect(page).to have_css "button[aria-pressed='false']"
+    end
+  end
+end