Building the Discourse JS app is very resource-intensive. This commit
introduces an `assemble_ember_build` script which will check the
existing content of the `dist/` directory and re-use the core build if
possible. Plugins will always be rebuilt.
For now, this functionality is only useful for multi-stage (i.e.
non-standard) Discourse deployments. But in future, this script may be
extended to pull the contents of the `dist/` directory from a remote
location.
