Single repo

This removes the monorepo / yarn workspaces setup in favour of a simpler to work on and maintain single repository.
All paths and interdependencies have been updated accordingly, so the app still works with yarn start.

A couple of things have been moved around:

• The graphql.js file inside the messages model has been moved into the graphql api.
• The downloadEndpoint files from book and book component version have been moved into the rest api.

The above were moved because they caused circular dependencies in the backend, breaking the app at certain points with empty results when requiring models. This is now fixed.

Apart from the restructure, there are no other changes in functionality.