Create and test a function insertNewRole

[jaytula]

This issue requires that #181 is resolved first.

We need a function to insert a new role into the roles table in the DB.

The functions should take this shape:

const insertNewRole = async (name: string) => {
  // Have fun
}

Return either the result of the insert query or null for when the role already exists.

• Function should be in db/roles.ts
• Also write tests to test in test/roles.test.ts