feat: slack notification on add feed form

[Alessandro100]

closes #763 Summary:

When the add feed form successfully adds a feed to the correct google sheet, it will send out a slack notification

Expected behavior:

When in DEV as a feed is added through the form, it should send a notification to the slack channel #slack-test-channel (private channel)

When in PROD the channel will be #internal-add-feeds

Testing tips:

Submit a test form with the preview url and you should see a notification in the #slack-test-channel

Please make sure these boxes are checked before submitting your pull request - thanks!

Notes Because we have a lot of control we can add high level information to the slack notification ex: GTFS or GTFS RT, is it from official producer, etc. Data that would be useful at a quick glance

• [ ] Run the unit tests with ./scripts/api-tests.sh to make sure you didn't break anything
• [ ] Add or update any needed documentation to the repo
• [X] Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• [X] Linked all relevant issues
• [X] Include screenshot(s) showing how this pull request works and fixes the issue(s)

[github-actions[bot]]

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