UI: Sidebar & header tweaks and fixes

Depends on !1953 

See the commits for more details, the messages mostly speak for themselves.

![Logo](https://gitlab.com/gitlab-org/gitlab-ce/uploads/ba088150165bb0a0aea1e894f3443f43/logo_tooltip.png)

![Sidebar](https://gitlab.com/gitlab-org/gitlab-ce/uploads/c1e8c09ac3bf489cdb9531eebea18c90/sidebar_tooltips.png)

Fixes #2928, #2071, https://dev.gitlab.org/gitlab/gitlabhq/issues/2575, and https://dev.gitlab.org/gitlab/gitlabhq/issues/2573

See merge request !1958