Implement method that reads vendor libs from core.json

* includes them at the beginning of initTemplate()

If this fails it will put an error message into log, but will continue loading (which result in a broken web UI)

@PVince81 I guess this was your idea ;)

@DeepDiver1975 @LukasReschke 
What about throwing an exception instead if the file cannot be `json_decode`d or not read?

Not sure what a running ownCloud instance without any JS would be good for ;-)