[ticket/13318] login_username needs to have the multibyte parameter set to true

This is needed for usernames with UTF-8 characters.

PHPBB3-13318
[ticket/13209] Fix expected request value of CPF language fields.

Multiple languages can be edited, thus the expected value is an array.

PHPBB3-13209
[ticket/13209] Gather language field values for boolean CPF's in all steps.

The user can move back and forth between all three steps and as such the lang
field data should be expected in all steps.

PHPBB3-13209
[ticket/13239] Ensure "Add files" button position is correctly calculated.

PHPBB3-13239
Merge pull request #3139 from prototech/ticket/13239

[ticket/13239] Ensure "Add files" button position is correctly calculated.
Merge pull request #3137 from prototech/ticket/13209

[ticket/13209] Fix loss of CPF language field values when switching steps
Merge pull request #3125 from lavigor/ticket/13318

[ticket/13318] login_username needs to have the multibyte parameter set to true