feat: invite members to slack after onboarding session 👖

[ramiAbdou]

Description ✏️

This PR makes an update such that the invitation to our Slack workspace only happens after attending an onboarding session, not just finishing the Member Profile onboarding flow.

• Moves the automatic Slack invitation after an admin from the workspace an admin uploads onboarding session attendance.
• Shows the "Book an Onboarding Session" card in the Home page.
• Removes the Slack and GitHub from "Important Resources".
• Removes the "Slack" step from the Member Profile onboarding flow.
• Fixes a bug with the seed script.

Type of Change 🐞

• Feature - A non-breaking change which adds functionality.
• Fix - A non-breaking change which fixes an issue.
• Refactor - A change that neither fixes a bug nor adds a feature.
• Documentation - A change only to in-code or markdown documentation.
• Tests - A change that adds missing unit/integration tests.
• Chore - A change that is likely none of the above.

Checklist ✅

• I have done a self-review of my code.
• I have manually tested my code (if applicable).
• I have added/updated any relevant documentation (if applicable).