fix gathering duplication bug

[xjlin0]

When auto/manually creating series of gatherings/attendances, duplication of gatherings start/end on UTC rather than local time zone will be accidentally created because of timezone ambiguity. This MR fix that and always use meet.infos['default_time_zone'] rather than users timezone since users can be in any timezone.

Query to find meets that does NOT have default_time_zone:

SELECT 
    id,
    slug,
    display_name,
    infos
FROM occasions_meets
WHERE is_removed = false
  AND (
    infos->>'default_time_zone' IS NULL 
    OR infos->>'default_time_zone' = ''
  )
ORDER BY id;