Refactor contract instance architecture

[b1s0ndev]

Context: Right now we instantiate the contract inside of the App.js and EventCreation.js files, but this is a bad design practice. Essentially we want to set this up so that we handle all of the contract instances in a state management library.

Tasks:

• Create a new file called contract.js by creating a new ./services directory
• We will want to require the contract instance in EventCreation.js (should be included at the top like: const {getInstance} = require('../services/contract')
• Inside contract.js should consist of all the necessary getters and setters for the contract and account instances.