[3.2][ticket/8064] Forum author sort using username_clean

+ Conversion of the solution to 3.1.x
+ Adding the migration file
+ Bug fix (for moderation)

Tests still missing.

About the migration file, we need to fill the column
(topic_first_poster_name_clean) while updating the db but it's not easy
and the migration could take a lot of time with thousands topics.

https://tracker.phpbb.com/browse/PHPBB3-8064

PHPBB3-8064
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