Make book-reivews pages more like meetings pages

* set a category in the frontmatter
* move the excerpt into the parts frontmatter and use the content/filter structure
* use a book_reviews helper to find relevant pages by their category
* update the layouts to do the right thing
Shuffle around HTML in the layouts

We can put header/footer/sidebar and all the boilerplate into the base layout and thus remove duplication between the others.
Remove unused helper method with_children_of

Also the ChildrenQuery object which was used by it.
Specify layouts for book-reviews and meetings in config.rb

We are able to start removing complexity from layout.erb and use custom layouts for meetings.  We're also able to remove the layout: frontmatter and specify it in config.rb so we don't forget and get the wrong layout for a new meeting or book-review.