Add pictrs fetch / upload to API

[dessalines]

History: we used to forward picture requests and uploads directly to pictrs, but now pass the requests through lemmy. Since these are passed through lemmy anyway now, we should add proper websocket and HTTP API actions like the rest of our actions.

This will also remove the need for nginx when developing.

[Nutomic]

Then we could make separate endpoints for uploading post image, user avatar or something else. This would also allow for a better way to implement #1118.