Store contexts as todos, not tags, step 2: port ContextRepository

Summary:
Port Akonadi::ContextRepository to the new serializer methods.
This required passing a DataSource to create(), which has impact all the
way up to the GUI, which now needs to request a data source from the
user when creating a context.

Reviewers: ervin

Reviewed By: ervin

Differential Revision: https://phabricator.kde.org/D19138