DEV: Swap out optipng with oxipng (#15013)

* DEV: Swap out optipng with oxipng

The oxipng binary has been added to our base docker image here:

244c9cb110

oxipng is a rust replacement for optipng that provides increased
performance and multi-threading. Checkout
https://github.com/shssoichiro/oxipng for more info.

* add instructions for installing oxipng

docs/DEVELOPER-ADVANCED.md

@@ -14,6 +14,12 @@ To get your Ubuntu 16.04 or 18.04 LTS install up and running to develop Discours
     sudo apt-get -yqq update
     sudo apt-get -yqq install software-properties-common vim curl expect debconf-utils git-core build-essential zlib1g-dev libssl-dev openssl libcurl4-openssl-dev libreadline6-dev libpcre3 libpcre3-dev imagemagick redis-server advancecomp jhead jpegoptim libjpeg-turbo-progs optipng pngcrush pngquant gnupg2
 
+    # oxipng
+    mkdir /tmp/oxipng-install && cd /tmp/oxipng-install
+    wget https://github.com/shssoichiro/oxipng/releases/download/v5.0.1/oxipng-5.0.1-x86_64-unknown-linux-musl.tar.gz
+    tar -xzf oxipng-5.0.1-x86_64-unknown-linux-musl.tar.gz && cd oxipng-5.0.1-x86_64-unknown-linux-musl
+    cp oxipng /usr/local/bin
+
     # Ruby
     curl -sSL https://rvm.io/mpapis.asc | gpg2 --import -
     curl -sSL https://rvm.io/pkuczynski.asc | gpg2 --import -

docs/DEVELOPMENT-OSX-NATIVE.md

@@ -225,7 +225,7 @@ In addition to ImageMagick we also need to install some other image related
 software:
 
 ```sh
-brew install jpegoptim optipng jhead
+brew install jpegoptim optipng oxipng jhead
 npm install -g svgo
 ```