Re-write Admin model

[bbenefield89]

File: ./src/models/Admin/Admin.ts

The Admin model is currently a stand-alone function that takes a Sequelize database connection as an argument and returns a model. This should become its own Admin class to fit in better with this projects OO approach.

• Should take a Sequelize argument in the constructor
• Substantiation should return a Sequelize model which will inherit all Sequelize model methods