Add add config validators

src/configloader/ValidateInstanceLink.go

@@ -0,0 +1,47 @@
+package configloader
+
+import (
+    "errors"
+    "net/url"
+    "net/http"
+    "strings"
+)
+
+// Validates a museum-digital instance URL to upload to.
+// The link must be a valid URL. As it can be expected that people will
+// enter broken values, the URL should be reduced to its hostname and
+// rebuilt from there.
+func ValidateInstanceLink(instanceUrl string) (string, error) {
+
+    parsed, err := url.Parse(instanceUrl)
+    if err != nil {
+        return "", err
+    }
+
+    // Check the hostname contains "museum-digital.de" or "museum-digital.org"
+    if !strings.HasSuffix(parsed.Host, "museum-digital.de") && !strings.HasSuffix(parsed.Host, "museum-digital.org") {
+        return "", errors.New("This tool only supports uploading to XYZ.museum-digital.de or XYZ.museum-digital.org")
+    }
+
+    // Check /musdb is available on the md instance
+    musdbUrl := url.URL{
+        Scheme: "https",
+        Host: parsed.Host,
+        Path: "/musdb",
+    }
+    resp, err := http.Get(musdbUrl.String())
+	if err != nil {
+		return "", err
+	}
+	if resp.StatusCode > 399 {
+        return "", errors.New("The museum-digital subdomain does not contain a musdb path")
+	}
+
+    // https should be assumed and enforced.
+    url := url.URL{
+        Scheme: "https",
+        Host: parsed.Host,
+    }
+    return url.String(), nil
+
+}