Add PHPUnit test for demo adapter
Use adapter to retrieve content list
Use adapter to retrieve content metadata
Use adapter to issue content
Use adapter to retrieve content resources
Use adapter to return content
Delete deprecated ContentHelper

The purpose of ContentHelper was to avoid implementing database queries
in DaisyOnlineService class. After refactoring and implementation of
DemoAdapter the class is deprecated.
Normalize variable name contentId in adapter and service code
Normalize annoucementId and qestionId variable names
Delete sessionUserId variable

DaisyOnlineService does not need to store user id anymore, this shall be
stored in the adapter if necessary.
Document the purpose of sessionUsername
Use adapter in session handling and establishment

If adapter returns false on sessionStart the client/device must
re-establish a session.
Require prior call to getContentMetadata before issuing content