cubicweb-timesheet #7632781 Provide the missing-activities on Ressouces as json [validation pending]

Reuse the code that is used to generate the missing-activities page, and provide a json view of the same data.

prioritynormal
typeenhancement
done in0.14.0
load1.000
load left0.000
closed by#2bcb5788f70d refactor ResourceMissingActivitiesBoard to introduce a view returning the same information as json. Closes #7632781
patchrefactor ResourceMissingActivitiesBoard to introduce a view returning the same information as json. Closes #7632781 [applied]