Scripts: get bootstrap.js and popper.js at build time

[chalin]

• Contributes to:
  • Ensure that versions of dependencies remain in sync #1182
  • Support non-CDN resources #605 by eliminating the CDN call to Bootstrap and making the CDN call to Popper at build time
• In preparation for Upgrade to Bootstrap 5.2 #470

[chalin]

As is mentioned in Components from the Bootstrap docs:

Tooltips and popovers [are used] for displaying and positioning (also requires Popper)

Tooltips are still working (they're used in the footer):

[LisaFC]

Looks good!

[chalin]

Thanks for the review. Merged so that I can more easily experiment with:

• Upgrade to Bootstrap 5.2 #470

[deining]

This commit broke docsy module installations. We need to add a mount so that bootstrap.js becomes available with module installations (see PR #1216).