Add filter cssrewrite

According to : http://symfony.com/doc/current/cookbook/assetic/asset_management.html#cookbook-assetic-cssrewrite

For keep link to image in CSS
minor #3859 Add filter cssrewrite (DOEO)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #3859).

Discussion
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Add filter cssrewrite

According to : http://symfony.com/doc/current/cookbook/assetic/asset_management.html#cookbook-assetic-cssrewrite

For keep link to image in CSS

Commits
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0210a35 Add filter cssrewrite