check js config validity as part of build

[MortenHofft]

Site owners will on rare occasions make changes to the site js configuration. e.g. site scope.
Not everyone is familiar with javascript. I would be nice to add a vlidataion step to the build

node --check _includes/js/config.js could be a good start. It will just check that the content has valid syntax

[MattBlissett]

Done.

https://builds.gbif.org/view/Hosted%20Portals/job/hp-swissnatcoll/1672/console looks to fail because of it.