Don't disable button to download emails

Rails automatically disables buttons when submitting a form. This works
fine most of the time: for AJAX requests, it enables them again after
the request is complete, and for non-AJAX requests, the button is
replaced by a new element when the new page loads.

However, there's an exception. When a request returns data so users can
download a fire, the request is not an AJAX one and the button is not
replaced. So users are left with a disabled button they can no longer
click.

So in this case, we aren't disabling the button after a user clicks it.

spec/system/admin/emails/emails_download_spec.rb

@@ -38,4 +38,11 @@ describe "Admin download user emails" do
       expect(file_contents).to match_array ["admin_news1@consul.dev", "admin_news2@consul.dev"]
     end
   end
+
+  scenario "Download button is not disabled after being clicked", :js do
+    visit admin_emails_download_index_path
+    click_button "Download emails list"
+
+    expect(page).to have_button "Download emails list", disabled: false
+  end
 end