Optimize image sizes and format

[ceolinwill]

Is your feature request related to a problem? Please describe.
Even though we limit image sizes when users are uploading them, they're still large and inefficient. Plus, sometimes we only need a thumbnail (e.g. for posts list). We shouldn't load the whole image.

Describe the solution you'd like
We should optimize all images from posts, topics, and groups. We should consider using formats such as WebP and create different image sizes (e.g. thumbnails).

Additional context
Posts can have multiple images and they can also come from external sources (e.g. image from links added to the references). When should optimize all those images and, when coming from external sources, we should copy and upload them to our storage bucket.