Allow users to filter display of location and variant in the interactive visualization and URL parameters

[huddlej]

Users should be able to select a subset of locations and/or variants to display in the interactive visualizations and link to those filtered views through URL parameters. For models with many locations, the on-hover approach to filtering variants isn't possible (the legend goes off the screen as you scroll down) and the on-hover approach doesn't allow permanent selection of specific variants or multiple variants.

[huddlej]

@rneher requested this functionality, too, asking for a way to click to select specific variants and keep them highlighted while scrolling across panels.