Implement the expand/collapse all feature on activity pages

This discussion originated on this issue: #79

We need a way to expand/collapse all cards shown in the activity pages view. The flow and the wireframe are detailed below.

After talking with @sheetaluk here's the flow we worked out:
1. If I have cards open already and I want the rest of them open, I can hit “expand all” and it will open all the cards.
2. Once I hit Expand all, The text changes to “collapse all”.
3. After a user hits “expand all” and all the cards have been opened, they can hit “collapse all” to collapse all the cards

The only thing I can think of that’s problematic with this approach is if a user hits “expand all” and the “expand/collapse” icon is at the top, there has to be an easy way for us to bring a user back to the top of the page. (I will write this up as a separate issue).

I'm also including Conor's mockups of this. You can find the svg file of the expand/collapse icon here: https://drive.google.com/open?id=0ByC8gz0BSv0tVWZGa1dieDZCNEk