Directory /pages contains user stuff, it belongs to /user : move it there

This should be automated during update if possible:

• check write permission & create directory, copy files there
• attempt to delete /pages (should fail most of the times)
• display notice explaining what's left to do for the user with a link to the wiki

Wiki document :

• explain what to do
• explain why it's better (all user & install dependant stuff regrouped into one directory)