redmine: refactor, cleanup, bug fix, and add functionality

- added package option to specify which version of redmine
- added themes option back in to allow specifying redmine themes
- added plugins option back in to allow specifying redmine plugins
- added database.socket option to allow mysql unix socket authentication
- added port option to allow specifying the port rails runs on

- cleaned up Gemfile so it is much less hacky
- switched to ruby version 2.4 by default as suggested by documentation http://www.redmine.org/projects/redmine/wiki/redmineinstall#Installing-Redmine
- fixed an annoyance (bug) in the service causing recursive symlinks
- fixed ownership bug on log files generated by redmine
- updates reflecting renames in nixos options

- added a nixos test