Improvements to image handling in posts

Doing a bit of brainstorming about how we can handle images in posts better.

- [ ] Add `width` and `height` attributes to rendered images to prevent content jumping as images load. ([Use this library](https://github.com/willwashburn/fasterimage))

- [ ] When a large image is included in a post (greater than a certain number of MB) treat it as an image upload - download a local copy, make a thumbnail, and wrap it in a lightbox so that you can click on it to view the full size.
- Probably do this as part of an Attachments extension
- Inspiration: https://github.com/discourse/discourse/blob/master/app/jobs/regular/pull_hotlinked_images.rb https://github.com/discourse/discourse/blob/master/lib/cooked_post_processor.rb

Start by reviewing the post-rendering path and the proposed Attachments extension, then compare the linked Discourse image-processing examples. The work is done when rendered images reserve their dimensions and oversized remote images are downloaded, thumbnailed, and presented in a lightbox, with the scope and threshold decided.

Written by the indexing model from the issue text.