Mini activities feed proof of concept

Throwing this up here prematurely so we can discuss the approach. It's pretty straightforward but just want to make sure that it makes sense and see if I'm missing anything. This also needs some styling, it's supposed to live in the grey footer area. I only added the Blog doc type and the Newsroom categories (as their own kind of pseudo doc types).

![image](https://cloud.githubusercontent.com/assets/1093373/5127198/a5f0cad4-70a0-11e4-928e-fd9724fe6223.png)

Maybe something like:

set activity_types = ['posts', 'press release', 'speech', 'testimony', 'op-ed']
for type in activity_types
      (generate snippet & add to a feed list)

Then you could pass in a restricted list to use instead of the default.  I don't know if there will actually be a need to restrict types, though -- there was just a line in the description of the offices page activity section about displaying <= 6 activities categories that made me think about it.