Updated sidebar style to span the whole height.
Updated mobile sidebar to allow scrolling
Changed header to stay at the top of the page.
Merge branch 'ui-fixes' into 'master'

Ui fixes

Fixes the ui bugs described in #925.

Also added the suggestion to keep the header at the top of the page.

Fixes #925

Some before/after screenshots:

- Border doesn't reach bottom on smaller pages
  - [Before](https://gitlab.com/uploads/gitlab-org/gitlab-ce/5096094d94/Screenshot_from_2014-12-27_19_49_24.png)
  - [After](https://gitlab.com/uploads/gitlab-org/gitlab-ce/551c1924e4/Screenshot_from_2014-12-27_19_51_43.png)

- Ability to always scroll 48px (visible by the scrollbar)
  - [Before](https://gitlab.com/uploads/gitlab-org/gitlab-ce/ea591bf7b3/Screenshot_from_2014-12-27_19_53_41.png)
  - [After](https://gitlab.com/uploads/gitlab-org/gitlab-ce/97df62b40d/Screenshot_from_2014-12-27_19_53_17.png)

- Mobile sidebar icons off screen (Visible by the scrollbar on the after picture)
  - [Before](https://gitlab.com/uploads/gitlab-org/gitlab-ce/b6d9a4d076/Screenshot_from_2014-12-27_19_49_05.png)
  - [After](https://gitlab.com/uploads/gitlab-org/gitlab-ce/fc52ac31ba/Screenshot_from_2014-12-27_19_48_16.png)

- Header fixed at top
  - [Before](https://gitlab.com/uploads/gitlab-org/gitlab-ce/5d74dc1ea5/Screenshot_from_2014-12-27_19_59_28.png)
  - [After](https://gitlab.com/uploads/gitlab-org/gitlab-ce/57c4c3ea7d/Screenshot_from_2014-12-27_20_00_05.png)

See merge request !269