Switch the menu buttons from images to text with css

This change would be better for accessibility and translations, and would
allow to add new menu entries more easily.

Signed-off-by: Remington Steed <rjs7@calvin.edu>
Use span instead of button, darker selected color

This commit changes menu items from 'button' tags to span tags since
those are more standard when using links (instead of forms). It also
adjusts the CSS as necessary to maintain the appearance. And it changes
the color of selected menu items so they are a little more visible.

Signed-off-by: Remington Steed <rjs7@calvin.edu>