Replace uglifier with terser

[6twenty]

This replaces Uglifier with the more modern Terser for minifying javascript, since Uglifier is only able to handle ES5 javascript and chokes on any modern syntax. Our plan to remove support for IE11 involves no longer transpiling our modern javascript down to ES5 syntax, so we need a corresponding minifier that's capable of understanding modern javascript. Terser also handles older javascript syntax just fine too, so this shouldn't yield any difference when minifying our older javascript code that's still written as ES5.

[6twenty]

Sorry - meant to target our own fork as the base branch for this one 🤦