fix(filters): support early return in modify_post and modify_feed

[shouya]

Previously, the user code is embedded in a function that looks like this:

async function modify_post(feed, post) {
  <USER CODE>;
  return post;
}

It means in the filter config, if the user wants to early return, they must call return post; explicitly. This is a little clumsy given we want to give the illusion that the user no longer has to explicitly return post or anything.

I hope to enable the user to write code like this in modify_post and modify_feed filter:

if (condition 1) {
  // do no modify post by early return
  return;
}

if (condition 2) {
  // delete post by assigning it null;
  post = null;
  return;
}

if (condition 3) {
  // alternatively, delete post by returning null
  return null;
}

// normally edit the post
post.field = 'new value';

Because now the return statement for user code has different semantics from the return statement of modify_post function, we must wrap the user code in a different function.

This pull request changes the wrapper we embedded user code to:

async function modify_post(feed, post) {
  const retval = await (async function(){ <USER CODE> })();
  if (retval === null) {
    return null;
  }
  return post;
}