Update popper.js from 1.x to 1.16.1-lts

https://www.npmjs.com/package/popper.js/v/1.16.1-lts

cf. https://www.npmjs.com/package/@popperjs/core for @popperjs/core 2.x.

Changelog

• https://github.com/popperjs/popper-core/releases/tag/v1.16.1-lts
• https://github.com/popperjs/popper-core/releases/tag/v1.16.1
• https://github.com/popperjs/popper-core/releases/tag/v1.16.0
• https://github.com/popperjs/popper-core/releases/tag/v1.15.0