Clean all generated files in order to avoid branch checkout failure + install missing smee-client

[joao-paulo-parity]

Prevents the error shown in https://github.com/paritytech/substrate/pull/9018#issuecomment-854389779

When I tried to deploy this branch, Probot logged a warning

WARN  (probot): Run `npm install --save-dev smee-client` to proxy webhooks to localhost.

That makes sense because smee-client is not listed as a dependency