Add field in config form to add or upload list of emails to be banned

Indranil Roy → made their first commit to this issue’s fork.

I have added the patch.

Thanks for the MR, @Indranil Roy I still see few of the things which should be added to make it work properly.

1. We can add a schema file.
2. We will have to add an update hook to update the configuration for existing mailbox users.
3. We will have to add blacklist_emails in the config file as well.

Thanks for the review @kunal_singh, Will surely look into it.