Handle incorrectly formatted activities

On a mypod.store user, the following Note was attached to the `/as/note` directory:

https://app.wafrn.net/fediverse/post/798afe71-3cf7-4002-8e76-ddac802e5bcf

When fetching it with a `Accept` header of `application/ld+json` or `application/json`, it returns HTML.

When getting the `/as/note` container, it throws the following error:

`FetchError: invalid json response body at https://app.wafrn.net/fediverse/post/798afe71-3cf7-4002-8e76-ddac802e5bcf reason: Unexpected token '<', "

Start by tracing how the /as/note container fetches and attaches the linked activity, reproducing the request with application/ld+json and application/json against the provided URL. Done means invalid HTML is not attached as a Note, and an already attached malformed item does not make the LDP container request throw.

Written by the indexing model from the issue text.