Default containers are always restored

[bllngr]

I'm using container-sync on multiple machines with different operating systems and Fx versions. I've heavily modified my containers, and removed all default containers.

Every few days (probably after using Firefox on a different machine), the default containers get restored. It seems like the have been synced at one point (probably after setting up a new profile using Firefox Sync, where the extension gets automatically installed and starts syncing immediately). Since then, they can't be removed from the list of synced containers and will be restored at some point.

This is related to #6, but with the additional problem that the default containers will be synced automatically on a fresh install.