Add support for pre/post update hooks in the PluginEditCtrl.

This allows users to intercept the update procedure from their own
controller to modify the pluginSettings before they are saved and
perform additional tasks like syncing state via a third party API.

public/app/features/plugins/edit_ctrl.ts

@@ -9,6 +9,8 @@ export class PluginEditCtrl {
   includedPanels: any;
   includedDatasources: any;
   tabIndex: number;
+  preUpdateHook: () => any;
+  postUpdateHook: () => any;
 
   /** @ngInject */
   constructor(private backendSrv: any, private $routeParams: any) {
@@ -24,20 +26,55 @@ export class PluginEditCtrl {
   }
 
   update() {
-    var updateCmd = _.extend({
-      pluginId: this.model.pluginId,
-      orgId: this.model.orgId,
-      enabled: this.model.enabled,
-      pinned: this.model.pinned,
-      jsonData: this.model.jsonData,
-      secureJsonData: this.model.secureJsonData,
-    }, {});
-
-    this.backendSrv.post(`/api/org/plugins/${this.pluginId}/settings`, updateCmd).then(function() {
+    var chain = Promise.resolve();
+    var self = this;
+    // if set, handle the preUpdateHook. If this returns a promise,
+    // the next step of execution will block until the promise resolves.
+    // if the promise is rejected, this update will be aborted.
+    if (this.preUpdateHook != null) {
+      chain = chain.then(function() {
+        return Promise.resolve(self.preUpdateHook());
+      });
+    }
+
+    // Perform the core update procedure
+    chain = chain.then(function() {
+      var updateCmd = _.extend({
+        pluginId: self.model.pluginId,
+        orgId: self.model.orgId,
+        enabled: self.model.enabled,
+        pinned: self.model.pinned,
+        jsonData: self.model.jsonData,
+        secureJsonData: self.model.secureJsonData,
+      }, {});
+
+      return self.backendSrv.post(`/api/org/plugins/${self.pluginId}/settings`, updateCmd);
+    });
+
+    // if set, performt he postUpdate hook. If a promise is returned it will block
+    // the final step of the update procedure (reloading the page) until the promise
+    // resolves.  If the promise is rejected the page will not be reloaded. 
+    if (this.postUpdateHook != null) {
+      chain = chain.then(function() {
+        return Promise.resolve(this.postUpdateHook());
+      });
+    }
+
+    // all stesp in the update procedure are complete, so reload the page to make changes
+    // take effect.
+    chain.then(function() {
       window.location.href = window.location.href;
     });
   }
 
+  setPreUpdateHook(callback: () => any) {
+    this.preUpdateHook = callback;
+  }
+
+  setPOstUpdateHook(callback: () => any) {
+    this.postUpdateHook = callback;
+  }
+
   toggleEnabled() {
     this.update();
   }