♻️ (server): Refactor User model field resolver

[tkhwang]

Description

Type of change

• Refactor follow/unfollow usecase.

• Use only followersCount and followingsCount instead of its lists.

• [ ] 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)

• [ ] ✨ New feature (non-breaking change which adds new functionality)

• [ ] 🐛 Bug fix (non-breaking change which fixes an issue)

• [X] ♻️ Refactor (non-breaking change which refactors code)

• [ ] 🩹 Chore (non-breaking change which changes the small things)

• [ ] 📝 This change requires a documentation update

Further to-do lists

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