Preview posts in stream

[#2108](https://github.com/diaspora/diaspora/issues/2108)

This will add a preview button to the publisher. When you press the button the current post will be presented in the stream. When you share the post the preview will disappear.

Just like the share button the preview button is disabled if there is nothing to post.

Currently not working:
- oEmbed [#2171](https://github.com/diaspora/diaspora/issues/2171)