[ticket/11881] Timezone migration can take a long time

PHPBB3-11881
[ticket/11881] Better split the timezone conversion into chunks; add test

PHPBB3-11881
[ticket/11881] Make sure user_timezone isn't converted twice

PHPBB3-11881
[ticket/11881] Limit to 500

PHPBB3-11881
[ticket/11881] Fix test filename

PHPBB3-11881
[ticket/11881] Move convert_timezones_test to migrator

PHPBB3-11881
Merge branch 'exreaction/ticket/11881' into develop-ascraeus

* exreaction/ticket/11881:
  [ticket/11881] Move convert_timezones_test to migrator
  [ticket/11881] Fix test filename
  [ticket/11881] Limit to 500
  [ticket/11881] Make sure user_timezone isn't converted twice
  [ticket/11881] Better split the timezone conversion into chunks; add test
  [ticket/11881] Timezone migration can take a long time