cubicweb-registration #4491860 registration emails should have HTTPS URLs when available [validation pending]

Plain text should be avoided at all costs.

prioritynormal
typebug
done in0.6.0
load0.500
load left0.000
closed by#b958f4fa3904 [views] Always generate HTTPS URLs if the instance supports said protocol
patch[views] Always generate HTTPS URLs if the instance supports said protocol (closes #XXX) [applied]