UI tweaks: separate mobile and desktop UI

As suggested in [1]
- time of a post is moved to the right
- action buttons are now only visible when a post is hovered or tapped
- on screens <768px the action buttons are 20px to make them more accessible
- on screens >768px we introduce extra whitespace between posts

[1] https://github.com/innoq/statuses/pull/119#issuecomment-64163097