Settings Page UI redesign

[MdSaifAliMolla]

closes #1375

[gnprice]

Thanks. It looks like you have a couple of other PRs open already:

• Get route-transition duration robustly,instead of hard-coding #1920
• KaTeX overline underline border #1919

Let's start with feedback on those. After you've successfully revised those to be highly reviewable PRs (https://zulip.readthedocs.io/en/latest/contributing/reviewable-prs.html), then you'll be more prepared to come back and do the same with this one.