Implement About Ghost page

Closes #3568
- Deleted html placeholders in client
- Added new grunt task, buildAboutPage, which 1)creates -contributors.hbs partial and 2) downloads contributor avatars
- buildAboutPage is called by anything that does an emberTemplates task
- Removed unused code from ghostpaths
Merge pull request #3575 from novaugust/about-ghost

About ghost page