Backend: Create limit, income, and expense models in limit-model.js, expense-model.js, and income-model.js files

[CT-cb]

Files are located in the src/Models folder in the backend repo.

Each model should be in the form of a Mongoose Schema. See the user-model.js file for a rough example.

Please include at least all of the attributes that are in the relevant models in src/old/db