Clicking on the "All subscriptions" link before any feeds have been loaded does nothing.

The exception is if feed pagination has finished without loading any feeds (i.e. the user is not subscribed to any feeds with unread entries). In this case the link can be clicked once pagination has finished.

The reason is that if we allow the user to click on the link before any feeds have been loaded, the entries list will be left blank which is confusing for the user. It's better to keep him from clicking on the link while it's not yet functional.
Moved the logic that decides if the "All subscriptions" link is enabled to a separate function, so that it can be reused.
Visually disable the "all subscriptions" link while it's not yet functional.

The link is not functional until at least one feed has been received from the server, or until feeds pagination finishes if no feeds are received at all.

This is intended to avoid the case in which the user could click on the link to read all entries before any feeds had been received; this resulted in a blank space being shown where entries should be, instead of showing the list of entries.

This complements changes in commits cbcd1c58c555511e2c11c9e26263e6e6fc7a42e4 f944c119b365abed2b1ad25dae511f56753659ac. Those commits actually disabled the link until it was functional; this commit makes no functional change, it just adds the bootstrap "disabled" class so the user can be aware of when the link becomes functional.