Autopaginate GitHub requests

* fix for fetching only 30 teams from the organization
* fetch 100 items per page and iterates to get all the results
Make detection of services team case-insensitive

* Update `GithubApi#find_team` to detect existing services team with
  lowercase name
* Addresses https://github.com/thoughtbot/hound/issues/454
* https://trello.com/c/q9z4wV19/398-activation-fails-when-repo-belongs-to-organization-with-services-lowercase-team
Add SECURITY.md document, linked from footer

Move all similar docs to `doc/` directory.