Twitter cards - Fixes #856

Implemented a method to add a Twitter Card in the Header class and added a card to the blog detail action.

This will be tested by @WouterSioen as I don't have time the next few days.

Note: you need to validate your Twitter Card before it can be used with [the validator](https://cards-dev.twitter.com/validator)
enter before the curly brace. We always start the body of a function on a new line.