Standardize meta tags for SEO, remove Flutie

Google rewrites page titles in search results pages:

https://support.google.com/webmasters/answer/35624?hl=en

Since we can rely on Google adding, for example, "thoughtbot" and
"podcast" to a page title for podcasts.thoughtbot.com in search results,
this commit simplifies our page title strategy.

This commit uses the same `yield(:name).presence || t('layouts.name')`
pattern for page title and meta description. `presence` is necessary
because `yield` without a `content_for` returns an empty string, not
`nil`.

I have heard from our designers that they don't believe the `body_class`
method is any longer the most efficient way to style pages.

Since we don't need Flutie's page_title or body_class methods, it can be
removed.