Renamed DataImport model to OpmlImportJobState.

This is more homogeneous with other job state models (RefreshFeedJobState, SubscribeJobState)
Most of the code for parsing an OPML file and enqueuing jobs to subscribe the user to the feeds there has been moved to the new OpmlImporter.import method.
Renamed OpmlImporter class to OPMLImporter class.
Fix tests.
Fix tests.
Fixed tests.