Refresh assetHash on theme activation (#7464)

closes #7423

- asset hashes have never been refreshed properly!
- Ghost(Pro)'s 1-theme-only limitation has been hiding this bug for 3 years 🙄

core/server/middleware/theme-handler.js

@@ -60,6 +60,8 @@ themeHandler = {
 
         // clear the view cache
         blogApp.cache = {};
+        // reset the asset hash
+        config.assetHash = null;
 
         // set view engine
         hbsOptions = {