Support bypassing the proxy for some Lemmy instances

[geneccx]

Since LemmyNet/lemmy#3421 was merged, Lemmy instances on 0.18.1 and newer allow cross-origin requests.

Notably missing is lemmy.world, which currently is broken as it sends double CORS headers:

> fetch('https://lemmy.world/api/v3/posts/list');

Access to fetch at 'https://lemmy.world/api/v3/posts/list' from origin 'https://wefwef.app' has been blocked by CORS policy: The 'Access-Control-Allow-Origin' header contains multiple values 'https://wefwef.app, *', but only one is allowed. Have the server send the header with a valid value, or, if an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

[aeharding]

Added fix for image upload. Otherwise LGTM!

(related bug: LemmyNet/lemmy#3567)