cubicweb #2788086 Move massmailing code to its own cube [validation pending]

Seen with Adrien, this code is(/was?) used in an obscure part of crm, it can (and should) go into its own cube.

prioritynormal
typeenhancement
done in3.17.0
load0.500
load left0.000
closed by#7da8339cd768 [web/views] Move massmailing to its own cube (closes #2788086)
patch[web/views] Move massmailing to its own cube (closes #2788086) [applied]