[C2] Pages: add slug edition, return to list and pagination memory

[ywarnier]

In the new "Pages" feature in administration, add a few improvements:

• Add a field to edit the slug (called "Friendly URL")
• Load pages using the slug (instead of the title) when the page has enabled=1
• Show a link to preview the page using the page id and "/preview" only for admins
• In the edit page, add a button to go "Back" to the list of pages
• In the list of pages, if I previoously selected pagination of 50 items per page, keep that setting when I go to edit and return

[christianbeeznest]

It is implemented in this PR 916de39

Thanks for confirmation.