Create Slack Slash command service

Gemfile

Gemfile.lock

app/models/project.rb

app/models/project_services/slack_slash_commands_service.rb

app/models/service.rb

...ojects/services/mattermost_slash_commands/_help.html.haml

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

lib/gitlab/chat_commands/deploy.rb

lib/gitlab/chat_commands/help.rb

lib/gitlab/chat_commands/presenters/mattermost.rb

spec/lib/gitlab/chat_commands/command_spec.rb

...odels/project_services/chat_message/build_message_spec.rb

...odels/project_services/chat_message/issue_message_spec.rb

...odels/project_services/chat_message/merge_message_spec.rb

...models/project_services/chat_message/note_message_spec.rb

...models/project_services/chat_message/push_message_spec.rb

...s/project_services/chat_message/wiki_page_message_spec.rb