GitHub integration: check repo can be queried before starting parsing

[epipav]

Changes proposed ✍️

• Github worker now checks if the repo is available with given token first and adds the repo to the endpoints array if repo is available.

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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[epipav]

baseIterator changes are already reviewed by @joanreyero Rest of the functionality (that checks if gh repo is available or not) is unit tested and tested out in staging as well.