A few comment/image improvements

[micahmo]

Pull Request Description

This PR makes a couple of changes to comments and images.

1. Comment contents are now in an AnimatedSize. This means when anything changes with respect to the height of the comments (an example is viewing the raw markdown), it will more nicely animate.
2. Add a solid color placeholder for images previews in markdown. This should make it more obvious that an image is loading instead of there just being a blank space in comments.
3. Defer SVG checking, which previously would cause a big delay (and a big jump in comment size, which is annoying when scrolling) before the image placeholder could take its place. Now we assume it's a regular image and only show an SVG if we find otherwise.
4. Remove the "unable to load" message (especially since this looked bad in the feed) and replace with a more generic placeholder for a "failed" image. (This also makes the SVG loading look less bad than an error message.)

Hopefully that all makes sense!

Hide whitespace.

Screenshots

Bad Image Placeholder

Before	After

Image Load

Before

image-load-in-before.mp4

After

image-load-in-after.mp4

[hjiangsu]

Thanks for doing this - the comment section jumping was also somewhat annoying for me, but I never had the chance to look into it further!

LGTM - just one comment!