remove duplicated role declarations

[jay-jlm]

Currently user role is declared in 2 places, which lends itself to bugs when refactoring code.

A better solution would be to have a single source of truth and use variable reference:

const roles = require('helpers/roles.js')

const UserSchema = new Schema({
 // [...]
  role: {
    type: String,
    enum: Object.values(roles),
    default: roles.ROLE_MEMBER
  }