refactor(Subscription): rename columns and adjust primary/foreign keys

[wescopeland]

This PR updates the Subscription table to get it more closely aligned with Laravel best practices.

A migration has been added which performs the following:

• Drop the combined [SubjectType, SubjectID, UserID] primary key.
• Add a id column as the new primary key.
• Rename all columns:
  • SubjectType -> subject_type
  • SubjectID -> subject_id
  • UserID -> user_id
  • State -> state
  • Created -> created_at
  • Updated -> updated_at
• Change user_id to an unsigned big integer.
• Add a unique constraint to [subject_type, subject_id, user_id].
• Add a foreign key from user_id to UserAccounts.ID.
• Rename Subscription to subscriptions.

Additionally, the Subscription and User Eloquent models have been updated.