add dataset notes

[cypressf]

Certain datasets have specific qualifiers for certain counties, like health data with estimated counties. We need a way to display which counties are "special" in the map, and a way to describe that to the user. We could use hatching, with a hatch in the legend and a special note.

We could let people add special notes in the database, and annotate data values with those notes.

[cypressf]

Dataset notes could appear when you hover over the county, in the tooltip itself. Instead of showing hatching, we could just show the note on hover, that way we don't need to come up with a separate hatching visualization for each county.