cubicweb-forum #2963510 Update modification date of a thread that has been commented [validation pending]

This matches what a forum thread really is, and will be useful to order thread list in a logical manner.

prioritynormal
typeenhancement
done in0.1.0
load0.250
load left0.000
closed by#a9669550e58a Update modification_date of ForumThread instances upon comments activity.
patchUpdate modification_date of ForumThread instances upon comments activity. [applied]