If alerts

We now have two types of alert. I've made them a component and will go around and update other alerts in apps to use the new ones. I know there were some issues with alerts in thorntree using hyperlinks and not working with a component so will look at that (still useful everywhere else).

I also removed a couple of old and pointless typography variables.

![image](https://cloud.githubusercontent.com/assets/814861/3808184/508fb23e-1c73-11e4-8516-ce037ce3d3f3.png)

Here, I didn't want to have all this ruby at the top of the alert haml file so I made a helper. Having done so though, I wonder if this approach could scale for components where we don't think it's necessary to use a component. Buttons for example. The reason i originally scrapped the button component was because you had to cater for the element being one of `button`, `span`, `a`, `input` as well as catering for all the variation classes, aria roles, data attributes and it seemed clear it would be more effort to use it than writing the classes. But if you had the classes outside the component like this then technically you could instead call:

```
%a{href: '/my-button-destination', class: "#{ui_button_classes({color: blue, size: small} my-button-modifier"} My button
```

Not sure if this is a good idea, just something that I realised after I'd done this when thinking about thorntree alerts. Any thoughts/blockers?