Create models and relationships

[bos21001]

• [x] Create Eloquent models for each table (Finances, FinanceHistory, Group, GroupUser)
• [x] Define relationships between the models (hasOne, hasMany, belongsTo, belongsToMany, etc.)

[bos21001]

Models and relationships were set in the commits below:

403af36 e857fa0 9a56c31 6cdd8c8