Responsive digest

This is a moderate update to the Digest plugin. It replaces the two modes (mobile and digest) with a single digest mode that is handled responsively on the client-side using CSS media width selectors and some small JavaScript hooked to the window resize event (to toggle the `mobile_mode` variable). A few other features were added as well; see below for a complete list of changes.

### Responsive Design

The plugin will now automatically alternate between the mobile and digest modes. Seamlessly switching between a one-pane view (feed list, article list, or article content) to a two-pane view (feed list & article list, or article list & article content). The breakpoint is a width of 769 pixels. This was chosen primarily because it's a common width that will allow tablets in portrait to show the single-pane view, but in landscape to show the two-pane view. Smartphones should show only the single pane view in either orientation (unless they have a really big display).

The `Hello, USERNAME` text in the header is hidden on widths <= 480 pixels. This prevents the greeting from overlapping the `Go back` button on smaller devices.

The `$_SESSION["digest_mobile"]` variable has been dropped as it's now handled exclusively on the client-side.

### Improved Article List Options

The article list now has buttons to handle bulk selections: `All`, `Invert`, and `None` are now available and select articles appropriately.

### Improved Article Options

The article content now has a Read/Unread button beside the existing Star and Publish buttons. This allows users to mark an article as unread if they want to come back to it later.

### Updated Fonts

The `font-family`'s have been updated to match TT-RSS's 1.12 regular version.

### Miscellanea

Fixed a bug when calling Scriptaculous JS library. It was calling the dragdrop addon which was never available (resulting in a 404 error) and also not even needed.