Truncate long request and response bodies before monitoring or logging

When an external request fails `HTTPService` [raises `ExternalRequestError` passing the failed `request` and `response`](https://github.com/hypothesis/lms/blob/db1874d1e4b9d9be373b6fabecfd2be0023140e9/lms/services/http.py#L90). The request and response are then logged to Papertrail via [`HTTPService.__str__()`](https://github.com/hypothesis/lms/blob/db1874d1e4b9d9be373b6fabecfd2be0023140e9/lms/services/exceptions.py#L58-L83) and also sent to Sentry and to the frontend by the [`external_request_error()` exception view](https://github.com/hypothesis/lms/blob/db1874d1e4b9d9be373b6fabecfd2be0023140e9/lms/views/api/exceptions.py#L105-L147).

When sending a request or response body to Papertrail, Sentry or the frontend we should truncate it to make sure we don't send too much data.

Note that we actually _want_ to send the entire body for debugging purposes. But just not if the body is so massive that trying to send the whole thing is going to cause problems. So we'll want to set the truncation limit quite high