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/814861/3808468/ebe8bccc-1c77-11e4-8f6c-d131a0dcc6f0.png)


I like this idea actually! The one change I'd make is to have a convention where the helper method name should be `ui_classes_` prefixed... so we would have `ui_classes_button`, `ui_classes_alert`, etc.

I also think the idea of these helpers is a good one. Is this something we could adopt for [the pagination component](https://github.com/lonelyplanet/rizzo/blob/master/app/views/components/_pagination.html.haml) for example? The more logic-agnostic the views are, the easier it will be to try adapt them to different scenarios (like rendering them as mustache templates /cc @chee).