[ticket/11875] Set default data type from MEDIUMINT (3-bytes) to INT(10) (4-bytes)

Use `INT(10)` data types for `user_id`, `post_id` and `topic_id` across all tables

[PHPBB3-11875]

  [PHPBB3-11875]: http://tracker.phpbb.com/browse/PHPBB3-11875 "mediumint(8) too small for post_id"