- right now, it loops through all packages serially, which isn't
effectively using multi-core machines
- by using `concurrently`, we can rely on it to use all the cores it
can, so this should dramatically speed up the bundling step
fixes https://github.com/TryGhost/DevOps/issues/99
- this inlines the `monobundle` script into the monorepo from an
external repo in order to avoid some caching issues we've seen
- it also makes it easier to maintain because you can change the script
alongside changes in the monorepo