Add Dark Mode color theme for Dashboard

[bensheldon]

Closes #974

• Updates Bootstrap to v5.3.1, which has "color modes" built in
• Adds 3 new SVG icons. I'm considering at which point I want to pull in the entire icon font (which is 121kb to pull in the woff2 package)
• First time creating a Stimulus controller, though the Dashboard has been vendoring it for a while. To avoid a flash of styling this also required embedding some javascript in the html head element.
• Replaces the "Live Poll" checkbox with a Bootstrap-themed "switch" which looks fancy.