BK-1122 Wrong links since removing the old reader app

Since we removed the old reader application some links got broken. This task contains a subtask related with the book cover image:

- https://dev.sourcefabric.org/browse/BK-1123

We needed a view to serve the book cover as legacy reader did. BK-1123 subtask is included in same commit because it was a little change and also it is a dependency of the main task.