Change default image_mode to proxy, remove deprecated option

[Nutomic]

I would also consider deprecating/removing the option StoreLinkPreviews as proxying can achieve the same purpose much more efficiently.

Besides we should enable image proxying on lemmy.ml to ensure that it works well.

[dessalines]

I'd also be fine with removing the StoreLinkPreviews, although I think we have some bugs / structural issues with how custom_thumbnails interact with these.

We should probably leave as draft until we make sure lemmy.ml doesn't have any issues with it, as well as any federation problems.

[Nothing4You]

#4874 seems like one of the things that probably should be addressed before proxying becomes the default

[dessalines]

This has been running stable enough on lemmy.ml for a while now, that I think we can probably merge (after confllicts are fixed).

Up to you whether you want to completely remove the StoreLinkPreviews option here, or do that in a separate PR.

[Nutomic]

Proxying actually works worse than storing previews locally, for example #5307 (which is why the proxy_bypass_domains option is needed). Also thumbnails dont take much space anymore thanks to downscaling. So lets keep both options available and just remove this deprecated field.