Fix channel and mention names showing ID in slack

[Rochet2]

Partially fixes https://github.com/UniversityOfHelsinkiCS/toska-silta/issues/2 Fixes:

• Stop sending garbage with names <@435345634543>, just send @SaSu or Sasu
• Stop sending garbage with channel names <#2355364543>, just send #keskustelua_discordissa or keskustelua_discordissa