intermittent bug: setlist popover nonresponsive + `Popper` not defined

[alxndr]

Expected behavior

Tapping a [setlist] date should open a popover with the concert's setlist.

Observed behavior

0.2.3-alpha8 and earlier, Discourse v3.3.0.beta1

Tapping setlist does not show popover.

JS error observed in the console:

Uncaught ReferenceError: Popper is not defined
    at tippy.js@5:1:24214

---

The https://unpkg.com/popper.js@1.16.1/dist/umd/popper.min.js dependency may not be loading?
...nope, popper + tippy both still getting loaded, and eventually Popper is set in window:

---

Found when testing #1 ... unclear what relationship between these two is... when Popper isn't defined, does quote behavior work? ... sorta; pressing q does work as expected, but selecting text does not show the popup with "quote" buttons.

[alxndr]

solved after 0.2.3-alpha21 #28