Refactor config/ directory

This supersedes #4141

* Get rid of early pathname requirement
* Use require_relative where approciate
* Drop unused files
* Imported new application.rb, environment.rb and environments/* from fresh Rails app
* Cleaned up boot.rb
* Load config in boot.rb
* Deduplicate environments/integration*.rb
* Move username blacklist into defaults.yml
* Ruby 1.9 Hash syntax everywhere
* Reorganize lib/diaspora/markdownify to match conventions
* Get rid of full path requires where possible
* Add dummy content to production section diaspora.yml.example to prevent warning
* Drop sqlite? method
* Move postgres? method into ConfigurationMethods
* Drop token authentication from User