Reorganization of the angularjs code related to showing/hiding read feeds.

Now instead of passing a boolean argument around, a boolean flag is set as soon as possible to indicate whether we want to load read feeds or not, and most methods just read the flag from the root scope instead of expecting a boolean argument.

This is the first step towards extending the functionality of this flag, so that it also controls whether to load read entries.
The "show read" and "hide read" buttons in the navbar control not only the display of read feeds, but also the display of read entries.

The "show unread entries" button has been removed, as its functionality has been absorbed by the "hide read" and "show read" buttons.
Simplified navbar buttons layout, moving buttons around a bit.