Handle `<hr>` horizontal lines

[gnprice]

We should handle the <hr> element.

This element means a "thematic break", shown as a dividing line; the name stands for "horizontal rule". It's the result of a --- line in Markdown.

Handling these means parsing them in lib/model/content.dart, and then styling them appropriately in lib/widgets/content.dart. For appropriate styling, see the Zulip web app's web/styles/rendered_markdown.css.

[jassirr]

@gnprice @chrisbobbe can I take this one up?

[jassirr]

@gnprice @chrisbobbe any update?

[Khader-1]

Instructive, but I imagine having the link to web/styles/rendered_markdown.css would give more context for readers.
Anyways, I am working on it!

Current progress: