Review responsive layouts (eg max-width)

[davelab6]

I chatted with @yuinchien today and she requested to review the page in Chromium Web Inspector's mobile/tablet preview modes and check how it works in various viewport sizes, and add some more responsive CSS rules to improve it - for example, on a desktop with a wide viewport the page lacks max-width rules that make its contents become very wide and unsightly.

[graphicore]

I improved some aspects here:

• max-width is now at 1900px.
• For the Chromium Web Inspector's mobile/tablet preview to work I had to add the tag <meta name="viewport" content="width=device-width, initial-scale=1" />, which I though I had already there, sorry for the inconvenience.
  I didn't add too many new responsive CSS rules, most of the existing ones weren't triggered in that preview mode due to the missing meta tag.