[WIP] JS Extender API foundation (#1468)

* Run extenders exported by extensions
* Add some basic extenders
* Patch Mithril as the very first thing so extension code can run safely
* Load the payload into the app before booting extensions
* Setup default routes before booting extensions

js/src/common/extend/Model.js

@@ -0,0 +1,41 @@
+export default class Routes {
+  type;
+  attributes = [];
+  hasOnes = [];
+  hasManys = [];
+
+  constructor(type, model = null) {
+    this.type = type;
+    this.model = model;
+  }
+
+  attribute(name) {
+    this.attributes.push(name);
+
+    return this;
+  }
+
+  hasOne(type) {
+    this.hasOnes.push(type);
+
+    return this;
+  }
+
+  hasMany(type) {
+    this.hasManys.push(type);
+
+    return this;
+  }
+
+  extend(app, extension) {
+    if (this.model) {
+      app.store.models[this.type] = this.model;
+    }
+    
+    const model = app.store.models[this.type];
+
+    this.attributes.forEach(name => model.prototype[name] = model.attribute(name));
+    this.hasOnes.forEach(name => model.prototype[name] = model.hasOne(name));
+    this.hasManys.forEach(name => model.prototype[name] = model.hasMany(name));
+  }
+}