slack: broken markup due to truncation when fields takes a long string

[yasunariw]

For commits pushed notifications, the main text is currently provided as a value under fields.

When this text becomes long, Slack truncates at an arbitrary point, and as a result often breaks the markup. To fix, should at least use the text attribute instead of fields, or better, switch to layout blocks as we're finding that the legacy fields tend to have unpredictable behavior.

instance: https://ahrefs.slack.com/archives/C03254B74/p1609909026258700