Refactor Association data class, repository, view model, explore screen and all related tests to get the association data through firestore (with categories)

[Zafouche]

Association:

• Add enum class AssociationCategory. The enum class should contain the display name.
• Add category field to Association data class.

Hydrate and Serialization:

• Refactor both of these files to work with the Association data class additions

AssociationViewModel:

• Refactor the necessary code
• Add a helper function that groups the associations by category when fetched.

Explore screen:

• Remove mock association list usage
• Use the association view model to fetch the data

Tests

• Re-adapt all the necessary tests related to any of the changes above