Design refinements - Events listings (mobile)

Some small screen design fixes from Ben W:

**Listings**

There were a couple of spacing issues on event listings.

Before:
![screen shot 2014-12-29 at 13 15 45](https://cloud.githubusercontent.com/assets/123386/5569934/c087c200-8f70-11e4-80ff-9e17553fc2ef.png)

After:
![screen shot 2014-12-29 at 15 39 05](https://cloud.githubusercontent.com/assets/123386/5569943/d6a0d2b6-8f70-11e4-9bc5-5636907d93a5.png)


**Filters**

There were a couple of spacing issues with filters on small screens.

Filters are now a separate component, rather than being coupled to the `events-block` component. Refactor also improves the display of the drop-downs on mobile when there is an active subcategory.

I think there's something smarter that could be done here by checking to see if we have active sub-categories and adding a modifier class, in order to tweak the layout when there are multiple drop-downs, but my Scala knowledge isn't strong enough to add that logic yet.

Before:
![screen shot 2014-12-29 at 15 34 28](https://cloud.githubusercontent.com/assets/123386/5569923/b055495c-8f70-11e4-873c-9c524e5f7c9b.png)

After:
![screen shot 2014-12-29 at 15 34 17](https://cloud.githubusercontent.com/assets/123386/5569930/b86fe886-8f70-11e4-86b3-304ee929f7ab.png)