allow access to page to order book page's children

Problem/Motivation

The documentation on d.org is arranged in a hierarchy of nodes, using Book module. In many cases, the nodes that are the children of a particular parent node are not in any sort of logical or clear order. Rearranging them would make the order more logical and comprehensible to readers.

Here is an example: https://drupal.org/documentation/modules/flag. The main documentation page for Flag module has a ton of child pages, which are in no particular order, and really ought to be.

However, changing the order of child pages like this requires all the weights to be set correctly in relation to one another. This requires each page to be edited. It's a slow and tedious job!

Proposed resolution

We need a single admin page with weights for all the child pages.

Book module provides an admin page with weights, at admin/content/book/%node, but:

- it's for the entire book. This would mean have access to reordering the WHOLE of the 'Structure Guide' in one go, which I don't think is a good idea. *I* wouldn't want access to that, for fear of breaking everything!!
- it requires the 'administer book outlines' permission, which is too strong.

What we need is a tab on a single book page, that allows reordering just its child pages. I feel we could grant access to that to users who can edit book pages (which is everyone).

There is a project which provides this functionality, written especially for this issue: https://drupal.org/project/book_child_order

Remaining tasks

* The docs team needs to agree this would be a good thing. DONE: #2128143: proposal: allow documentation editors to order child pages of a book page.
* the project needs to be reviewed and deployed on d.org

User interface changes

Book nodes that have child nodes will gain a new tab, where the order of the child nodes can be rearranged.