Add License Finder.

License Finder checks the licenses of the gems and npm packages we are using to
make sure they do not conflict with the project. The config/license_finder.yml
contains the whitelisted licenses and the ignored Gemfile groups.

To use license_finder, simply run license_finder.

This commit also changes the package.json use be dependencies instead of
devDependencies. This causes a conflict with LicenseFinder.