Reading bootcamp information from GitHub and caching for local build

1. Added `bin/get_bootcamp_info.py` to read a list of GitHub bootcamp pages' URLs from `config/bootcamp_urls.py` and create `./_bootcamp_cache.yml` (which contains information about the bootcamps for use in the build process).
2. Modified `./index.html` and `bootcamps/index.html` to link either to a local bootcamp page or a GitHub bootcamp page.
3. Moved other configuration files into `./config` directory to reduce clutter.
4. Modified `./Makefile` to rebuild bootcamp info cache.
5. Removed local bootcamp pages for Harvard and STScI bootcamps (since their info can be fetched from GitHub).