Account deletion not working

[aforge]

When the user attempts to delete an account from the Tinodios, the application returns "Action failed: malformed (400)".

Steps to reproduce the behavior:
0. The problem can be easily reproduced in the local setup (e.g. bring up a single-instance Tinode server locally per https://github.com/tinode/chat/tree/master/docker/docker-compose#full-stack). Tinode server will be available locally at port 6060.

1. Start a Tinodios client and log in as alice with password alice123.
2. Go to 'Profile -> Account & Security'.
3. Click on 'Delete account' and confirm the action.
4. See error

Expected behavior
The account should be deleted cleanly from both the server and the client.

Additional context

1. Tindroid demonstrates the correct behavior of the account deletion feature and can be used as a reference.
2. Tinodios is missing the implementation of the Storage.deleteAccount() which is supposed to purge the account data from the client's local storage.

[aforge]

Fixed #160.