Recreate the OHI assessment page visualizations

[robyngit]

The ohi-assessments page was created using leaflet and R-markdown. In our meeting today we decided to do the following:

• Recreate the map using D3
• Make the polygons on the coastline more prominent (right now they are not immediately noticeable)
• Move the content into a regular markdown page in Hugo

Content required (@Melsteroni @gclawson1)

• The original RMD file (if possible)
• The shape files for the markers and coastline polygons

Note: we also decided that the individual assessment pages (e.g. https://ohi-science.org/assessment/Spain-Gulf-of-Cadiz) are not required, since they contain the exact same information that's included on the main page

We need this visualization to be available to use as a partial in the assessments page layout, as well as a shortcode to use in the OHI+/WHAT IS OHI+ page. See #125

[robyngit]

The OHI+ map is made here: https://github.com/OHI-Science/ohi-science.github.io/tree/master/assets/maps