Move html pages to a package

That way I can separate out HTML for the public-facing report system and
Nidus sync.

htmlpage/response.go

@@ -0,0 +1,13 @@
+package htmlpage
+
+import (
+	"net/http"
+
+	"github.com/rs/zerolog/log"
+)
+
+// Respond with an error that is visible to the user
+func respondError(w http.ResponseWriter, m string, e error, s int) {
+	log.Warn().Int("status", s).Err(e).Str("user message", m).Msg("Responding with an error")
+	http.Error(w, m, s)
+}