refactor the build

@zendesk/linksf 

Fixing some stuff I was always unhappy about.

* remove intermediate Handlebars step for token interpolation
* condense app + admin builds
* use grunt-env instead of homebrew .env
* use .env.dev and .env.prod to avoid switching key names
* remove cruft comments

Before this can be merged, I will port the deploy to Node so we can remove the Ruby dep altogether (along with work in https://github.com/zendesk/linksf/pull/228).