Fine-tune the formatting of the slider to make it look nice

[mciethan]

Currently, we have a shiny widget that allows us to choose from a handful of slider "skins" shown here: http://ionden.com/a/plugins/ion.rangeSlider/skins.html

However, we may want to make our slider more customized. Here are some starting resources for that: https://stackoverflow.com/questions/36138124/control-the-appearance-of-a-sliderinput-in-shiny https://www.w3schools.com/howto/howto_js_rangeslider.asp

In my opinion, our slider should show what year it's currently on, and we should also keep a set of tick labels along the bottom, to show what years are available, but I'm not in love with the beginning and end labels above the slider, which seem redundant if we are doing ticks along the bottom. If we can get rid of those beginning and end labels, I'm generally agnostic about more specific formatting stuff like widths, colors, shapes but open to others' inputs on making the slider look good.

[mciethan]

the Square style seems to be well-liked enough, and the ticks on it are gray and unassuming compared to the default, which is good