Minor adjustments API Mattermost

app/controllers/projects/mattermost_controller.rb

...els/project_services/mattermost_slash_commands_service.rb

config/routes/project.rb

lib/mattermost/command.rb

lib/mattermost/session.rb

lib/mattermost/team.rb

spec/fixtures/mattermost_new_command.json

spec/lib/mattermost/command_spec.rb

spec/lib/mattermost/session_spec.rb

spec/lib/mattermost/team_spec.rb

...roject_services/mattermost_slash_commands_service_spec.rb