Wiki: Increase font size and free space to make it more readable

This bugged me for a while. It's not perfect now, but I like it much better. Feedback welcome :)

![op-wiki-space](https://cloud.githubusercontent.com/assets/188986/3147952/07ca4194-ea5b-11e3-867e-8a59fe37d763.gif)

The one with the larger font and more free space is the new version.

See: https://www.openproject.org/work_packages/8605