Minor change in log wording.
WIP: perform OPML import in a single worker, instead of queuing as many workers as feeds in the OPML.
Extract code that runs on error and success finish conditions when importing subscriptions to separate private functions.

Better maintainability this way.
When enqueuing a job to subscribe to a feed, adapt to changes in arguments to SubscribeUserWorker in 787ff762b295f2299f5458af4f7e9e9c9022d2d7

SubscribeUserWorker now is only enqueued when the user requests a new subscription. It is never part of an OPML import, and the arguments related to OPML import have been removed.
Remove folder_id argument from SubscribeUserWorker#perform

It is not necessary to pass the folder id anymore. Since recent changes, SubscribeUserWorker is enqueued only when the user manually requests a new subscription; this means that a nil was always being passed in this argument. Better to get rid of it.

This makes a private method so simple that it's been removed and its code incorporated to the main perform method.
Renamed private function.
Worker validation was missing a return statement if the validation failed.
WIP: adapt tests to recent changes in subscribe and OPML import workers.
Fixed variable name in log statement.
Fixed variable name in log statement.
Reorganization of test suite.
WIP: adapt tests to recent change in subscribe and import subscriptions workers.
Adapted tests to changes in subscribe and import opml workers.
Fixed test to adapt to recent changes in arguments to SubscribeUserWorker#perform