Horizontally center story tiles in viewport

Horizontally center story tiles in the viewport so they look
better on narrow displays.

Make the container slightly wider so we can fit 3 columns of tiles
on small displays.

Fix the margin on the story count and view selector elements so
they don't get covered up on very narrow displays.

Addresses #927
Dynamic toggling of sticky filters

Using a width-based media query to toggle sticky behavior doesn't
handle the case where the window is somewhat wide, but short.
This is exactly what happens when a mobile device is held in
landscape mode.

Instead, in the JavaScript scroll event handler, check to see if
the height of the filter container is less than a specified
percentage of the window height and only stick the filters if they
won't cover too much of the window.

Addresses #927
Nicer horizontal padding on filters

Put them in a centering container instead of explicit pixel padding.

Addresses #927
Responsive styles for explorer list view

Addresses #927
Hide map view icon on narrow displays

There's a bit of work needed to make the map view show up on narrow
displays, so for now, just hide the icon.

Addresses #927