[ticket/11871] Allow backslash in classnames for Modules

PHPBB3-11871
[ticket/11871] Fallback to int ID of the module, when creating links

Firefox is currently unable to correctly copy a urlencoded \ so users will be
unable to post links to modules. However we can still fallback to the id
instead of the name, so we do that in this case.

PHPBB3-11871
Merge pull request #1739 from nickvergessen/ticket/11871

[ticket/11871] Allow backslash in classnames for Modules