Support SVG previews in markdown

[micahmo]

Pull Request Description

I was investigating issues I've seen displaying some images (most often in instance/community sidebars). It turns out that a common culprit is SVG files, which ExtendedImage cannot render.

This PR adds the ability to check whether an image is an SVG (getting only the headers, so the http request is lightweight), and if so, using a different widget to render the SVG (from jovial_svg) in markdown. This works in post bodies, comment bodies, and sidebars. It doesn't affect when a post link references an SVG. Like the web, that is still treated as an external link. It also doesn't allow SVGs to be tapped and opened in an image viewer.

I also enhanced the error messaging for when images legitimately don't load.

Note: I picked jovial_svg over the seemingly more popular flutter_svg because the latter failed to render the first thing I threw at it! 😊

Issue Being Fixed

Inability to render SVGs.

Screenshots / Recordings

Ignore the oversized Datadog image and render overflow. I would like to fix that separately!

Before

svg-before.mp4

After

svg-after.mp4

Checklist

• Did you update CHANGELOG.md?
• Did you use localized strings where applicable?
• Did you add semanticLabels where applicable for accessibility?

[hjiangsu]

LGTM! Just a couple of comments