Github webhooks star activities now use starred_at

[epipav]

Changes proposed ✍️

• Github webhooks were not returning a starred_at date previously, that's why we were using the current timestamp as activity timestamp for star activities coming from webhooks. This was causing duplicate activities between the graphql activities (which returns the actual star timestamp) and webhook generated ones.
• Now this field is returned from webhooks, and we start using this timestamp for star activities.
• For unstar activities we're still using current timestamp as activity timestamp.
• The tests that are using the whole api request cycle was failing bcs redis didn't exist in tests - adds a redis instance for the test setup.

Checklist ✅

• Label appropriately with type:feature 🚀, type:enhancement ✨, type:bug 🐞, or type:documentation 📜.
• Tests are passing.
• New backend functionality has been unit-tested.
• Environment variables have been updated
  • Front-end: frontend/.env.dist
  • Backend: backend/.env.dist, backend/.env.dist.staging, backend/.env.dist.staging.
  • Configuration docs have been updated.
  • Team members only: update environment variables in Password manager and update the team
• API documentation has been updated (if necessary) (see docs on API documentation).
• Quality standards are met.
• All changes have been tested in a staging site.
• All changes are working locally running crowd.dev's Docker local environment.

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