Delete datablock storage data on account deletion

[snickell]

Probably we just need a line in delete_accounts_helper.rb to manage this.

See:

• delete_accounts_helper.rb: https://github.com/code-dot-org/code-dot-org/blob/2a02fd1aefe55f98eb454876def9713e716de5da/lib/cdo/delete_accounts_helper.rb/#L73-L77
• Test: datasets_controller_test.rb: https://github.com/code-dot-org/code-dot-org/blob/2a02fd1aefe55f98eb454876def9713e716de5da/dashboard/test/controllers/datasets_controller_test.rb/#L34-L36
• Test: delete_accounts_helper_test.rb: https://github.com/code-dot-org/code-dot-org/blob/2a02fd1aefe55f98eb454876def9713e716de5da/dashboard/test/helpers/delete_accounts_helper_test.rb/#L53-L56

[cnbrenci]

This one unrelated? Test: datasets_controller_test.rb: https://github.com/code-dot-org/code-dot-org/blob/2a02fd1aefe55f98eb454876def9713e716de5da/dashboard/test/controllers/datasets_controller_test.rb/#L34-L36