ContestExporter: fix bug preventing export to archive

Fix issue #146.
CWS: fix bug in task description page

Don't assume that User.primary_statements contains an element for every
task name and use an empty list if it doesn't.

Fix issue #147.
YamlImporter: fix bug preventing script from doing anything

Since "not None == True" the script thought that DB access always
failed.
DB: add Dataset
DB: add SubmissionResult
DB: add UserTestResult
grading: Job
grading: get_task_type
grading: get_score_type
grading: update to use SubmissionResults
services: update get_submissions to get_submission_results
services: add function to get all to-autojudge datasets for a task
ES: loads of changes
SS: loads of changes
CWS: loads of changes
AWS: add handlers for Datasets
AWS: fix Manager and Testcase handlers
AWS: update to use SubmissionResults
AWS: allow to see submissions on a certain dataset
AWS: fix and improve the client interface

Correct the messages shown in the worker and queue statuses.

Add more useful links.

Show textareas for score type parameters to make them easier to edit
(until we find a way to wrap them up nicely).