Signer: Remove ?interactive argument, add `impoprt_secret_key` function

The name of the argument was confusing enough that this bug entered zeronet:

(prefixes [ "submit" ; "proposals" ; "for" ]
 @@ Client_keys.Secret_key.alias_param
   ~name: "delegate"
   ~desc: "the delegate who makes the proposal"
(*
   ...
*)
    Client_keys.public_key_hash
      ~interactive:(cctxt :> Client_context.io_wallet)
      src_pk >>=? fun (src_pkh, _) ->

The documentation says "while importing" so ~first_import:… will be more clear.