fixes 1957 Add option to send user chatmessage instead of dm

Fixes #1957 (closed)

• I added the option in config/config.exs
• created a new module lib/pleroma/user/welcome_chat_message.ex
• Added it to the registration flow
• I added tests
  • I also broke up the test for the welcome message so that each 'type' (dm, chat, email) is tested seperatly
• added to the cheatsheet
• added to the config/description.ex
• added to the Changelog.md