Viewer: make prev/next buttons stay put and enable/disable rather than appear/disappear. Tweak styling. Add a (x/x) section counter below prev/next buttons. Note: in this commit the title attribute on the prev/next buttons is not present because handlebars is totally kicking my butt with some behavior that seems really strange to me but is probably down to a misunderstanding of either how it works or the structure of context data being passed in. (#518)