cubicweb-timesheet #3114997 Improve missing activities URL [validation pending]

It would be convenient to be able to specify a CWGroup name at the same place as a user login in the missing activities helper URLs.

prioritynormal
typebug
done in0.9.0
load0.100
load left0.000
closed by#8d9f76649298 Allow using a group name in the missing activities URL and order result by login.
patchAllow using a group name in the missing activities URL and order result by login. [applied]