Inject mapping definition into form view

[LouTerrailloune]

Injecting the mapping into the view (and maybe the mapping name too) will allow us to customise the form widget rendering.

For exemple, in my case, I store pictures in 2 locations, one is publicly accessible, another one is not (pictures are accessed using liip/LiipImagineBundle). The url of the preview picture in the form contol is different for the 2 cases, but I have no way to known which one to use if I only have the picture metadatas.