✨ (stacked bar) hide single-item legends

[sophiamersmann]

Hide single-item legends for stacked bar charts. This is space-saving for mobile especially.

For stacked area charts with a "Change entity" button, the currently selected entity is shown in the title.

Before	After
https://ourworldindata.org/grapher/ace-north-atlantic-hurricanes	http://staging-site-stacked-bar-hide-legend/grapher/ace-north-atlantic-hurricanes
https://ourworldindata.org/grapher/tsunami-deaths	http://staging-site-stacked-bar-hide-legend/grapher/tsunami-deaths