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

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1. [x] query GH API for top contributors between releases ([via](/novaugust/top-gh-contribs))
2. [x] Create hbs partial for user list from 1 @ `core/client/templates/-contributors.hbs`
3. [x] Download avatar for each user, and move to `client/assets/img/contributors/<name>`
5. [x] Insert ghost config (DB, Version, Environment, Mail