Fix non-unique assignmentId

AMT will sometimes give assignments that have been released to a new worker using the same assignment id.
This turns the assignmentId field in the database into a unique key that combines assignment and worker id to avoid duplicate entry errors.
Merge pull request #1 from annacoenen/master

Fix non-unique assignmentId issue
Fixed markup.
Modified database so key is always unique (more elegant version of anna's fix)