Add high-priority-prs action (gatsbyjs#13848)

* Add high-priority-prs action

* Remove unused dep

.github/actions/high-priority-prs/Dockerfile

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+FROM node:10-slim
+
+# Labels for GitHub to read your action
+LABEL "com.github.actions.name"="high-priority-prs"
+LABEL "com.github.actions.description"="Pings slack to say \"HANDLE THESE PRS!\" :-D"
+# Here are all of the available icons: https://feathericons.com/
+LABEL "com.github.actions.icon"="activity"
+# And all of the available colors: https://developer.github.com/actions/creating-github-actions/creating-a-docker-container/#label
+LABEL "com.github.actions.color"="white"
+
+# Copy the rest of your action's code
+COPY . .
+
+# Install dependencies
+RUN yarn
+
+# Run `node /index.js`
+ENTRYPOINT ["node", "/index.js"]

.github/actions/high-priority-prs/README.md

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+# high-priority-prs action
+
+Ported from @KyleAMathews [gatsby-pr-bot](https://github.com/KyleAMathews/gatsby-pr-bot/). [Read more about it on Kyle's blog](https://www.bricolage.io/bulding-a-slack-bot-to-help-handle-large-numbers-of-prs/)
+
+Run locally [using act](https://github.com/nektos/act): `act -a "high-priority-prs"`
+
+See below for the original README.
+
+---
+
+Sends Slack message every so often with important PRs to respond to.
+
+This is an experimental effort to help with identifying the PRs that most need responses — initially breaking them up into two queues — one for those that have never seen a response from core maintainers and those that have had commits since the last comment so need a new review.
+
+The goal is make sure every contributor has a great experience and to prevent valuable PRs from not getting in.
+
+## Setup
+
+`yarn install`
+
+Change the slack channel ID to point to your own personal channel while experimenting (find that by [opening up slack in the web](https://gatsbyjs.slack.com) and copying the ID looking thing.
+
+If testing changes, you can also write out the GitHub API response to a file called data.json and then change code in `process.js` to load
+that file instead of waiting for API response from `index.js`. This speeds up iteration on design.
+
+You can test the output by copy/pasting the CLI output to https://api.slack.com/tools/block-kit-builder