Support deleting all messages of a private chat

Fixes #20.
3 jobs for 20-delete-all-messages in 2 minutes and 4 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #6285
pycodestyle

00:00:38

passed #6286
pylint

00:00:45

passed #6287
pytest

00:00:40

93.0%