Add Airbrake link in error mails

Add Airbrake link to error emails. Needs testing and review.

Pros:

* The link is added to the email the user sends us. This allows us to quickly see what caused the issue and also see other information e.g. user agent
* No static error pages, which means no need to translate the error page strings.

(possible) Cons:

* The error page is not static anymore, which means that there MAY be a situation where we can not display an error at all. For example, if there's an error in the ErrorController, then we are not able to show the error page. However, this approach is recommended by Valim, so it should be ok, (http://blog.plataformatec.com.br/2012/01/my-five-favorite-hidden-features-in-rails-3-2/ See num. 3)