Remove the M2M for Works

[Neraste]

The M2M relationship using a custom intermediary object is too confuse. It leads to a weird access to works in a song JSON representation:

{
  "works": [
    {
      "work": {},
    }
  ]
}

The intermediate object is not a real object, so its name is sometimes hard to understand, e.g. songworklink_set.

Instead, we should use a real WorkLink object with two FKs.