Only active apps should be compiled in dev server

[KrisJordan]

Rather than scanning to find all apps and treating each as an entry point as a part of the webpack server, we should consider using only the most recently modified app (and touch each app as we load it in the dev server).

[KrisJordan]

The other motivation for this is not breaking the whole environment if you have a syntax error in an old file. Start working on a new file? That's where we compile.