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)

* [x] have text be like a sentence: »shared **test** with Tom« (including share icon, filename in bold, and avatar left next to friend username)
* [x] use icons for actions on the left
  * green + added
  * red x deleted
  * blue circular arrows changed
  * black/grey shared icon
  * black/grey renamed icon
  * black/grey moved icon
  * …
* [x] don’t show own username or avatars, only say »You« (missing in the mockup)
* [x] multiple files not in a list, but in text: »Photo, Document, 9 more …«
  * hover over »9 more…« shows a tipsy tooltip with the other files
* [x] show only filename, full path only on hover in a tipsy tooltip
* [x] group by actions and dates (this is done already I believe?)

That’s basically it. What do you think?

### Open bugs:
* [x] .txt previews are generated with a white background, which is now displayed on white background instead of gray background as before [see](https://github.com/owncloud/apps/pull/1769#issuecomment-43059181)
* [x] it's too hard to guess which date belongs to which activity when your screen is a bit bigger [see](https://github.com/owncloud/apps/pull/1769#issuecomment-43059181)
* [x] when you share something with someone (or receive a share), the avatar should be shown left next to the name of that person. (Do not show the avatar of the current user!) [see](https://github.com/owncloud/apps/pull/1769#issuecomment-43061711)
* [x] what is that »$2a$08$ebx4UI…«? [see](https://github.com/owncloud/apps/pull/1769#issuecomment-43063514)
* [x] big previews only for images for now, the others don’t work too well
* [x] layout is borked on long lines (I’ll fix that) [see](https://github.com/owncloud/apps/pull/1769#issuecomment-43063514)

Fixes #1676