Hide vote choices for in-progress votes

```
<@ekneuss> can't we hide votes until voting phase is over ?
<@salathe> I don't see why not
<@Felipe>  just have to patch the voting module :p
```
This PR implements the suggestion above, to hide the choices that people made while a vote is in progress.  A voter can see their own choice, but no-one else's.  

After voting closes, the normal behaviour is resumed; showing all votes.

![Poll with votes hidden](http://i.imgur.com/T3Dx5.png)