Fixing issues with filter and nav dropdowns

This addresses a few frontend style issues.

1. Adding a select box to the filter list causes the select dropdown to display behind the filter dropdown. Changing the z-index solves this issue.

2. There is no `Clear` button for filters. I added an `<a href="./">Clear</a>` link to strip off any query params, but perhaps there is a better way to handle the link location.

3. The filter dropdown is being cut off by the footer when the page height is small enough. This is fixed two ways. I made the footer sticky so that it will never appear higher than the bottom of the browser window. I also changed the container's clearfix to not use `overflow:hidden` to clear floats.

4. There was a bug in chrome with the height of the nav dropdown items. A fixed pixel height solves this.