feat(rxFloatingHeader): Floating header for tables

Closes #619

Major kudos to @glynnis and @Droogans
In reviewing this again I realized that font size for the `input`, as well as button font size/size are larger for the floating header than elsewhere on the site. 

I tooled with the CSS in my browser while comparing it to the Cloud Servers page, and it seems like the change in size is being applied by wrapping the table in a div with a `.table-filters` class, combined with a `.filter-box` class on the `input` itself. The `button` sizes are being applied by `.table-filters button` -- see https://github.com/rackerlabs/encore-cloud-ui/blob/master/app/views/servers/Servers.html#L21-33.

Sorry I didn't catch this the first time around! :coffee: