Fix formatting issue with slack truncation

Slack now truncates messages at 8000 bytes, but this causes some problems.

1. The truncation does not pay attention to the formatting, so blocks that
   were opened with ``` are not properly closed (and thus not formatted)
2. The server automatically linkifies anything that looks like a link before
   it truncates, so the actual number of useful bytes that the message is
   truncated at is less than 8000 (the client strips the links in a ``` block
   itself)