provide a better title on topic pages for crawlers

Meta: [Will posts show up on Google](http://meta.discourse.org/t/will-posts-show-up-on-google/1184/10)

- [x] allows any view to provide a custom title
- [x] if a title is provided, the name of the site will be appended at the end (like it's done on StackOverflow)
- [x] set the topic title as the title of the topic page for better SEO