Make the code smoother by adding the machine user as the only access token calling the API

[FionaDL]

The code could be improved by only using one access token to access the GitHub API.

Since it is necessary to use the machine user's access token for a Heroku scheduler, it is also a good idea to also use the machine user for all API calls.

Expectation: Change the access token being used to make calls to the API in the issues sync worker and pull requests sync worker.

Also: Update the README to explain to new Dash users that they will need to create a machine user and add the Environment Variable for the API access token of the machine user.