only call _afterSync if sync actually happened

fixed endless recursion in 2fhotellicht where additional models where synced in _afterSync
fix tests: add missing implementations of getImageDataOrEmptyImageData for trl
Merge branch '3.7'
Merge remote-tracking branch 'github/master'
fix javascript depdencies

- own package for mustache
- own package for raven js (not required by default)
- commented out jQueryTransit for now
fix dependency
add description to composer.json