Add help link to header.

- people may need help from any page
- it is not obvious that help is under Dashboard:
    - when I first used GitLab, I clearly remember spending some time looking for the help.
    - the help URL is just `/help`, and not `/dashboard/projects` or `/dasboard/issues`, indicating that in the mind of the community, Help is not a part of `/dashboard`
- GitHub does it, so its probably a good design choice and GitHub users will be home

**Screenshots**

![screenshot from 2014-04-30 12 26 03](https://cloud.githubusercontent.com/assets/1429315/2840240/f8e06eba-d051-11e3-9b5a-77ecc9fc24b8.png)

![screenshot from 2014-04-30 12 26 22](https://cloud.githubusercontent.com/assets/1429315/2840242/fe0f87fe-d051-11e3-8cc7-513ea6cef9a2.png)

Hover still perceptibly changes link color on dark theme. On white theme, like before this PR link color is unchanged on hover.

![screenshot from 2014-04-30 12 26 28](https://cloud.githubusercontent.com/assets/1429315/2840243/01f5bc76-d052-11e3-990c-95be9bb2bc38.png)

**Details**

- made help bold and clear on the dark theme to make it more visible and coherent with the icons on the right
- made `header-font` used in the `.title` "Diaspora / Diaspora Title" a bit larger or else help would be larger than it

**Questions**

If you like this, how about also adding a **Blog** link on another PR? Very important stuff, and GitHub puts it up there.