Allow admin to create public deploy keys that are accessible to any project.

CHANGELOG

app/controllers/admin/deploy_keys_controller.rb

app/controllers/projects/deploy_keys_controller.rb

app/models/deploy_key.rb

app/models/deploy_keys_project.rb

app/models/user.rb

app/views/admin/deploy_keys/index.html.haml

app/views/admin/deploy_keys/new.html.haml

app/views/admin/deploy_keys/show.html.haml

app/views/layouts/nav/_admin.html.haml

app/views/projects/deploy_keys/_deploy_key.html.haml

app/views/projects/deploy_keys/index.html.haml

config/routes.rb

db/migrate/20150327122227_add_public_to_key.rb

db/schema.rb

spec/models/deploy_keys_project_spec.rb