charm upgrade during a new release does not upgrade the gui source

During the charm upgrade process we should check if the version of the GUI deployed and built is < the one in the sources directory. If so, we should perform the upgrade by either manually building (if the config is set to stable or local) or using the config changed hook by setting the version string to the new one for the user.

The end goal is that we don't monkey with user's versions of the GUI just because the charm itself was updated, but that when we release a new version of the GUI that the charm upgrade will help users update accordingly.