Airflow 2 - Make it compatible

Hey there, 

I noticed that Airflow 2 is now the main supported base-line for Airflow (Airflow 1 will become deprecated soon). It seems that flask is no longer the main supported UI which results in an error message. Would it be possible to make this extension compatible with Airflow 2?

```
airflow-web ModuleNotFoundError: No module named 'flask_admin'
```