Allows book and first child creation in OG group context

Currently, OD works well in OG groups for already created books that have at least one child.
- The Edit books tab appears only when at least a book is define fir this group
- Clicking on a defined book that has not yet a child
+ fires a message "Books need to have at least one piece of content in them in order to work correctly with the outline designer
+ opens a form for creating a child wiht the correct outline (child of the selected book), but without the group context

It would be great if
1) the "Edit book" tab is always available
2) a button "add book" is avalailable under this tag
3) the warning message is suppressed
4) the child page applies correctly the current book context

As creating books in the restricted context of a group is very relevant, it would be great if these enhancement where implemented.