Bug 832269 - Clean up calendar styles.

This change request implements the minimum changes necessary to support updates to the modify event flow in calendar.

In order to do this in a way that I thought was re-useable, without too much JS - I needed to modify building blocks. If we are unable to land the building blocks changes, then we will need to copy/paste a lot of it into calendar, and I wanted to avoid that.

This patch introduces changes to buttons.css and allows a wrapping <span> tag around inputs for styling. We have support for anchors, but it seems wrong to wrap an anchor around an input. Here is an example of the implementation:

```
 <span role="button" class="icon icon-dialog">
  <input type="date" />
</span>
```