Open/Close Cases, only allow updates on open cases

Case.jsp
(/=) only show add comment field if the case is open
(+) show open/close button as appropriate

CaseServlet
(/=) reorganize to prepare for better error handling
(+) implement open/close
(/=) better check valid username & check if modification is ok

HomeServlet, SearchServlet, …
(/=) change id to caseId for naming consistency

CaseDatastore, CaseDatastoreInterface
(+) implement open/close
(/=) rename addCaseNote -> insertIntoCaseNote
(/=) insertIntoCaseNote locks on the class object to avoid race
condition on the next caseNoteId
(+) Stub: insertIntoCaseDetail

DatastoreInterface, CaseDatastoreInterface
(+) insertIntoCaseDetail

ModelObject:
(/=) better handle boolean fields (sometimes the system wants to return
an Integer by default, force it to return a boolean if the field is of
type boolean)