Reusing the existing `IssuableBaseService#filter_params` which uses
the policies to determine what params a user can set, and which values
it can be set to.

This also removed the need for the seperate call to
`IssuableBaseService#ensure_milestone_available`.

The `Issues::BuildService` does not suffer from this because it limits
the params that are assignable to the `title`, `description` and
`milestone_id`.