Following Management: existing email subscriptions occasionally fail to load

[bluefuton]

When loading /following/edit with a soft refresh or by navigating to it in a new tab, sometimes the email subscriptions for an existing feed can completely fail to load.

The feed subscriptions, post email subscriptions and comment email subscriptions are all provided by the endpoint /read/following/mine. It appears that, if the FeedSubscriptionStore already has all the feeds and the Reader does not initiate a new fetch from the endpoint, the email subscription stores simply do not get populated.

cc @koke, who originally noticed this issue on the OS X desktop app.