[ticket/12641] Use jQuery's trim() method. IE8 does not support the native one

PHPBB3-12641
Merge pull request #2529 from prototech/ticket/12641

[ticket/12641] Use jQuery's trim() method. IE8 does not support the native one.

* prototech/ticket/12641:
  [ticket/12641] Use jQuery's trim() method. IE8 does not support the native one