Make MR dicussion and changes tabs more noticable

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'improve-mr-tabs' into 'master'

Improve merge request page UX

Increase Discussions and Changes tabs size. Added count of changes and comment to it

Before and after:

![Screenshot 2014-05-27 12.08.42](https://dev.gitlab.org/uploads/dzaporozhets/gitlabhq/31fe70c9be/Screenshot_2014-05-27_12.08.42.png)