CALS-199: Add mention-support to slack-notifications

[joakimen]

Summary

Adds support for mentioning users and groups when pipelines fail.

Description

• Add SlackMention class: Responsible for formatting users, groups and special mentions into the Slack API required format. Table-driven tests exercise valid/invalid format inputs.
• First green->red only: Users are only mentioned on the first pipeline failure that occur, not subsequent ones.
• Keep handler slim: Keep formatting and validation logic outside of lambda handler code, as handlers are harder to test than regular project code

TODO

• Test functionality in sandbox environment

[joakimen]

@sirius mulighet for en review?