dev-api: add developer portal, OpenAPI spec, and Flatenbadet case study

- New /developers/ page with API docs, SDKs, pricing, use cases
- OpenAPI 3.0 spec for Orders, Missions, Photos, Analytics
- Case study: Glasskiosken i Flatenbadet — complete ROI analysis
- Updated /order/ with recurring missions and frequency dropdown

services/frilans-payout/node_modules/xtend/mutable.js

@@ -0,0 +1,17 @@
+module.exports = extend
+
+var hasOwnProperty = Object.prototype.hasOwnProperty;
+
+function extend(target) {
+    for (var i = 1; i < arguments.length; i++) {
+        var source = arguments[i]
+
+        for (var key in source) {
+            if (hasOwnProperty.call(source, key)) {
+                target[key] = source[key]
+            }
+        }
+    }
+
+    return target
+}