Library fails to load with cookies disabled

When cookies are disabled, browsers (tested in Chrome and Firefox) deactivate localStorage as well.

In this case, the way we check for localStorage availability (`!! localStorage`) throws a `SecurityError` ("The operation is insecure.") so that the whole library fails to load.

This PR introduces a central method on the remoteStorage instance to check for localStorage availability that handles the error.

While pairing on this with @skddc we also refactored the feature loading code a bit. We renamed some variables and methods to hopefully be more descriptive and extracted some parts into separate methods.