search: fix server-side date behaviour

OffsetMinutesFromUTC was being passed incorrectly by the web app
(getTimezoneOffset if UTC - Timezone, we wanted Timezone - UTC), now
fixed. This value is now used if given in comparisons. Times are
truncated to minute-deep precision, and Any date comparison ignores
empty date fields (i.e. a unix timestamp being 0 or a time.Time.IsZero()
== true).

activitysort.go

@@ -249,7 +249,7 @@ func matchTimeAsQuery(query *badgerhold.Query, q QueryDTO) *badgerhold.Query {
 	}
 	criterion := andField(query, "Time")
 	query = criterion.MatchFunc(func(ra *badgerhold.RecordAccess) (bool, error) {
-		return q.Value.(DateAttempt).Compare(ra.Field().(time.Time)) == int(operator), nil
+		return q.Value.(DateAttempt).CompareWithOperator(ra.Field().(time.Time), operator), nil
 	})
 	return query
 }