Add Slackbot

[gordlin]

Changes

• Adds a Slack bot that posts every time a new PR is created in the storylines-editor repo, giving details of the PR as well as an AI-generated summary of its description.
  • The AI summary uses the Pegasus summarizer model, which seems to do a good job and requires significantly fewer compute resources than a generalist model (ala LLAMA 3.2).
  • The Python transformers library is used to handle the model.

Notes

Links to the PR types list pages ("PR: Active", "PR: Geo", etc.) are hard-coded, and will need to be manually changed if the names of the types are modified.
QA Testing
Please test against main branch https://ramp4-pcar4.github.io/ramp4-pcar4/main/demos/enhanced-samples.html

Testing

1. Read the YAML file, and ensure the workflow is correct.
2. Once merged, the bot should summarize all new PRs in the storylines-editor repo.

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This change is 

[github-actions]

Your demo site is ready! 🚀 Visit it here: https://ramp4-pcar4.github.io/storylines-editor/slackbot

[szczz]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @gordlin)