rename output for css assets

In a production environment, when trying to dump the assets, for some reason if the name of the output is backend.css (same of the layout.html.twig), it is overwritten by this and the blueimp gallery is lost.
In the dev environment that doesn't happen.
Renaming the output to something else fixed it for me.
Modify _variant.html.twig in order to be able to see images at generated pdf invoices
Added parameter 'true' to imagine filter
Merge pull request #1176 from fourlastor/patch-1

rename output for css assets