Small UI improvements to make project sidebar more neat

* make sidebar blocks more compact
* make clone panel smaller for better aspect ratio
* make `forked from` section cleaner

Screenshot:

![Screenshot_2015-05-06_16.42.12](https://gitlab.com/gitlab-org/gitlab-ce/uploads/69723db619ae7ae4d9551f6340d45384/Screenshot_2015-05-06_16.42.12.png)

cc @rspeicher

See merge request !617