New welcome page (homepage) for Sourcegraph.com

Created by: sqs

Overall, build on our current homepage (https://sourcegraph.com/welcome, or https://sourcegraph.com when not signed in) to make it "feel more like a developer tool" and make other changes in response to feedback I've heard:

• Update some of the copy.
• Do not use custom scrolling effects. Developers generally don't like these, and I've gotten specific feedback to this effect.
• Support both color schemes (light/dark) and do not use custom colors, so that it's easy for anyone to update this page without needing CSS changes.
• Make modals into separate pages with their own URL.
• Replace codeintellify snippet on main page (but not code intel page) with demo video or screenshots, because the codeintellify snippet doesn't convey "how this is valuable" or "when would I use this".
• Fix title casing (-> sentence case).
• Feature browser extension more prominently.

I meant to get around to this around the holidays but didn't get to it.

Screenshots