Allow user to customize default Dashboard page

Renames the "Design" profile page to "Preferences" and adds a field to customize the default Dashboard page:

> ![Screen_Shot_2015-06-11_at_11.12.53_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/b5282a3be7861d1148528c6bc9e7a0e0/Screen_Shot_2015-06-11_at_11.12.53_PM.png)

See merge request !778