Box Navigation Links: Implement rollover text on the image navigation buttons

Inside the card image div add text and style, add highlight so text is not directly on image. Example of image rollover (2nd section): http://pulse-it.glitch.me/learn.html

The dev team will need more specifics on the rollover, including how will text be styled, whether anything is animated, how would text not be on the image, etc. This is the vision for the container of the text.

Text that goes inside the "rollover styling" container is determined by https://github.com/codeforboston/mattapan-mapping/issues/102.