Improve for SEO

[stefanjudis]

https://www.dropbox.com/s/e1t6p9haplpcv72/Screenshot%202015-04-13%2019.53.44.png?dl=0

[marcobiedermann]

• [x] ~~Add unique Title~~
• [x] ~~Add unique meta description per page~~
• [ ] Add Open Graph tags
• [ ] Add Facebook tags
• [ ] Add Twitter Card tags
• [ ] Add Mobile specific meta tags
• [x] ~~Add title attribute to tags~~
• [x] ~~/articles: change title attribute to alt attribute on twitter images~~
• [x] ~~/videos: add alt attribute to thumbnails~~
• [x] ~~/slides: add alt attribute to thumbnails~~
• [x] Navigation: change span to strong tag
• [ ] ~~Add h1 to subpages~~
• [ ] Add schema tags to articles

~~Crossed things~~ are done already and in master.