Great simplifies jumbotron and page title handling for pages and blog po...

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Removed the redundant source/layout.erb and partials/blog_layout.erb. Changed the variable names in _jumbotron from headline & subheadline to use title & subtitle from page frontmatter. Doing so allows content_for doesn't have to be called per page. Also means that partial doesn't have to be called per page with parameters. Content creator can just set title & subtitle in frontmatter and be done. Yield_content Jumbotron moved out of layout to page layout. Blog layout was also cleaned up. Moved all the meta tags from layout.erb to _header partial. Removed the author meta tag from _header and all pages & posts because I don't think we have a Google Authorship strategy yet.