Consider SVGs instead of PNGs for analytics images

[MattBlissett]

I've fixed the small error in the analytics that was causing SVG graphs to be a different size compared to the PNG ones.

Using them directly (without going through Thumbor) would avoid us having to drop the Thumbor cache when new analytics are released, and give crisp graphs even on my 4K screen.

(In writing this, I notice the font is serif instead of sans-serif. I'll look at fixing that later.)

[MattBlissett]

PNG:

SVG:

Comparison as I see it:

[MattBlissett]

It would really make the release process for analytics simpler if these SVGs were used instead of the PNGs.

The current PNGs thumbnails still appear fuzzy on both Firefox and Chromium on Linux and MacOS on hi-definition screens.