Update design of blockquotes in content

[gnprice]

The alignment and spacing of blockquotes is getting an update. We should apply that update.

References

• Sibling issue: Update design of lists in content #1492
• Web PR (covering also lists): markdown: Adjust list, blockquote styles to spec. zulip#34529
• Web issue (covering many other things too): UI redesign: message typography, including mention pills zulip#22022
• Design thread: #design > updating lists, blockquotes to spec