Sharkey instances fail to give username. #2

Sharkey responds to https://<instance>/api/v1/statuses/<id>/context with context, but 404s on https://<instance>/api/v1/statuses/<id>/.

Instead it seems to have the info we are after based on my curl requests with curl 'https://<instance>/api/notes/show' -H 'content-type: application/json' --data-raw '{"noteId":"<id>"}'.

What the username resolver can do is fallback to that one, and if successful also return a more sharkey-compatible post url. Because the [Linked to this post] link will also be broken if using sharkey, which instead of /@<username>/<postid> uses /notes/<postid>.

This should be doable before v2, as we don't break any public api's from the v2 rc's and this internal api change doesn't change behaviour.

Whether this should mean v2.0.0 is cancelled and bumped to a v2.0.1-rc is... something I need to think about.