[WIP] Activity app design

:construction: 

@nickvergessen let’s use this for working on the Activity app design. Essentially we should move to an easier to read list view. Here’s a basic mockup of how it should look:
![owncloud activity app mockup](https://cloud.githubusercontent.com/assets/925062/2775213/1d61d056-cac3-11e3-8e85-5034e5bc3122.jpg)

* have text be like a sentence: »shared **test** with Tom« (including share icon, filename in bold, and avatar left next to friend username)
* use icons for actions on the left
  * green + added
  * red x deleted
  * blue circular arrows changed
  * black/grey shared icon
  * …
* don’t show own username or avatars, only say »You« (missing in the mockup)
* big previews only for images for now, the others don’t work too well
* group by actions and dates (this is done already I believe?)

That’s basically it. What do you think?