present: Scale down slides on smaller displays

On mobile and tablets, it was very difficult to view slides because the
slides were not designed to be smaller than 1250x750.

This adds a function to the JS that uses CSS scaling to make the slides
fit on smaller displays.

Fixes golang/go#21643

Change-Id: I68e9e2c1274aaf6396bf01d19ca023cddf76e2ec
Reviewed-on: https://go-review.googlesource.com/60270
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Francesc Campoy Flores <campoy@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Francesc Campoy Flores <campoy@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
2 files changed