Centralize link URL validation

internal/link/url.go

@@ -0,0 +1,26 @@
+package link
+
+import (
+	"fmt"
+	"net/url"
+)
+
+func ValidateHTTPURL(value string) error {
+	parsed, err := url.Parse(value)
+	if err != nil {
+		return fmt.Errorf("must be a valid URL")
+	}
+	if parsed.Scheme != "http" && parsed.Scheme != "https" {
+		return fmt.Errorf("must use http or https")
+	}
+	if parsed.Host == "" {
+		return fmt.Errorf("must include a host")
+	}
+	if parsed.RawQuery != "" {
+		return fmt.Errorf("must not include a query string")
+	}
+	if parsed.Fragment != "" {
+		return fmt.Errorf("must not include a fragment")
+	}
+	return nil
+}