Pass the page to show sponsors for to the show_sponsors helper

This is because current_page is likely not the correct page to start looking for the "sponsors" part, so we have to tell it where to look.
Always render the filter type of any page parts
Move the lrug-railsconf-part to 2006/ where it belongs
Do the yearly indexes via "dynamic pages" in the sitemap

This means we can get rid of the static files in each dir.  We can also get rid of the "percent-b-percent-y-archives" folder.  Finally we remove the category: 'meeting' tag from the meetings/index, we'll keep the category for the meeting pages themselves.
Pass all page parts through 'erb' as well as their filter

Also provide self as context while rendering so we can also call our helpers etc in the parts
Provide page_title helper

This allows us to take the title from either the frontmatter or a title set via content_for.  We prefer content_for to frontmatter.
Set meeting years as a variable in config

This lets us use it elsewhere, for example in the custom sidebar for meeting pages.  We also use the content_part_exists? / render_content_part pattern we established in sponsors to deal with allowing custom sidebars.
Make the rss template be based on category, not slug

Also back away from using the query API in favour of standard ruby enumerator stuff.
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.