Added unit test for the SubscribeUserJob.

In the future this background job will likely also be used when a user requests adding a subscription. Some changes will probably be necessary, though.
WIP: ImportSubscriptionsJob will not do the actual subscribing of the user to feeds present in the OPML.

ImportSubscriptionsJob will just read the OPML, create any folders necessary, and enqueue instances of SubscribeUserJob to perform the actual subscribing.

This is very necessary to be able to run the import process in memory-constrained environments like a tiny EC2 instance. The monolithic process ran into an out of memory condition and was killed abruptly, never being able to finish. It is expected that this new way of importing feeds with smaller jobs will be less memory-intensive.