Merge branch '352-conversation-screen-has-too-much-padding' into 'master'

Improve conversation screen graphics with less padding

This is how the conversation screen looks with this MR:

![1](https://code.briarproject.org/akwizgran/briar/uploads/3078636c93f7edc6b07034ad808ffc7c/device-2016-08-26-105325.png)
![2](https://code.briarproject.org/akwizgran/briar/uploads/fe0dcd3b1cbc6593aabed6faab1801d7/device-2016-08-26-105425.png)

This MR also replaces more raster graphics with vector graphics.

Closes #352

See merge request !298