Almanac.httparchive.org: Move simple PNG icons to SVG

[bazzadp]

As a follow on from #738, the below PNGs don't scale well on large screens, or when zoomed in. As they are all simple (or have Vector versions for GitHub/Twitter) we should move them to SVG format.

• [ ] discuss.png
• [ ] results.png
• [ ] queries.png
• [ ] dropdown-arrow.png
• [ ] close.png
• [ ] menu.png
• [ ] blog.png
• [ ] github.png - Official link to Vector icons
• [ ] twitter.png - Official link to SVG icons.

Note some of these need a hover state version (the first 3 - but the dropdown-arrow, blog, github and twitter would also benefit from having this so maybe can solve at same time?).

[catalinred]

I'll add this comment as a reminder to myself and will get back when I'll have a plan on "svgify" the assets.