Improve style, add more tests

changelogs/unreleased/zj-slash-commands-mattermost.yml

lib/api/helpers.rb

lib/api/services.rb

lib/gitlab/chat_commands/base_command.rb

lib/mattermost/presenter.rb

spec/lib/gitlab/chat_commands/command_spec.rb

spec/lib/gitlab/chat_commands/issue_create_spec.rb

spec/lib/gitlab/import_export/all_models.yml

...odels/project_services/mattermost_command_service_spec.rb

spec/requests/api/services_spec.rb