MastoAPI: Support `pinned` parameter in Account api
This is part of #109 (closed) but gets its own issue because it is a real bug.
When you use the pinned=true parameter, Mastodon versions > 1.6 will return only the pinned posts. As we don't support this yet, we'll return all posts, so the client will think all posts are pinned. We claim to support 2.3.3, so this is a bug.
The easiest quick fix is to just return and empty list when there parameter is there and worry about actually supporting it later.