Notify CI failures on Slack

[waiting-for-dev]

Summary

We use the circleci/slack orb to notify whenever a CircleCI job fails. All jobs are included.

Currently, the CircleCI context has been configured to point to the #ci-notifications channel on Solidus' Slack workspace, where we have created the required Slack app.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• [x] I have written a thorough PR description.
• [x] I have kept my commits small and atomic.
• [x] I have used clear, explanatory commit messages.

The following are not always needed (~cross them out~ if they are not):

• ~[ ] I have added automated tests to cover my changes.~
• [x] I have attached screenshots to demo visual changes.
• ~[ ] I have opened a PR to update the guides.~
• ~[ ] I have updated the README to account for my changes.~

[mergify[bot]]

It looks like this PR is missing a label to determine the type of change it introduces. The maintainer should add one of the following labels:

• bug for bugfixes.
• enhancement for new features and improvements.
• documentation for documentation changes.
• security for security patches.
• removed for feature removals.
• infrastructure for internal changes that should not go in the changelog.

Additionally, the maintainer may also want to add one of the following:

• breaking for breaking changes.
• deprecated for feature deprecations.

Once the correct labels have been set, simply remove the needs changelog label label from this PR so I can merge it.