cubicweb-timesheet #7972532 error-prone edition of activity on a closed budget [validation pending]

currently if you attempt to edit an activity on a closed budget, the budget doesn't appear in the vocabulary list, hence the value is set arbitrarily to the first one in the list if you dn't see this and save. (i.e. the edit form is unusable)

priorityimportant
typebug
done in0.14.0
load left0.000
closed by#59af64486585 ensure current activity's budget appears in the list, avoiding undesired change.
patchensure current activity's budget appears in the list, avoiding undesired change. [applied]