Send email to challenge hosts only when the worker reload is complete and successful

**Current Scenario**: Whenever a challenge hosts updates an evaluation script or the test annotation file then the email for reloading is sent to them immediately after the upload of annotations file or the evaluation script file which gives a wrong sense to challenge hosts that the worker is reloaded but actually it is not because it takes 2 mins on AWS to reload it successfully.

**Fix**:
1. Send the email to challenge hosts immediately after upload saying that worker reload is started and it will take 2 mins to complete and they will be notified again when it is successfully reloaded.
2. Send another email to challenge hosts when the worker reload is successful on AWS.