Create models and perfom validation

[cherelemma]

In this PR, I created the models according to the ERD diagram given in the project requirements. Also I perform validation and testing the models.

Models created

• Users
• Categories
• Entities
• Categoriry_entities

Validation performed

• User Name - presence, length: { minimum: 1, maximum: 50 }
• Category Name - presence, length: { minimum: 3, maximum: 60 }
• Entity Name - presence, length: { minimum: 3, maximum: 60 }
• Amount - numericality and greater than zero
• Icon - presence

Testing prepared

• Users
• Categories
• Entities
• Categoriry_entities