Make the app name configurable even after the project is built (#731)

### What problem does this PR solve?

Make the app name configurable even after the project is built #730 

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

web/src/conf.json

@@ -0,0 +1,3 @@
+{
+  "appName": "RAGFlow"
+}