Fix some NPM bitrot

[bobdenotter]

Fixes the build error in latest Bootstrap update:

  Compiled with some errors in 33.25s

 ERROR  Failed to compile with 2 errors                                                                                  13:21:28

 error  in ./assets/scss/bolt.scss                                                                                       13:21:28

Module build failed (from ./node_modules/sass-loader/dist/cjs.js):
SassError: Undefined variable.
   ╷
20 │   @each $color, $value in $theme-colors-rgb {
   │                           ^^^^^^^^^^^^^^^^^
   ╵
  node_modules/bootstrap/scss/_root.scss 20:27     @import
  assets/scss/vendor/bootstrap/bootstrap.scss 5:9  @import
  assets/scss/vendor/vendor.scss 4:9               @import
  assets/scss/bolt.scss 3:9                        root stylesheet

Fixes the deprecations in Flagpack, like these:

: Using / for division outside of calc() is deprecated and will be removed in Dart Sass 2.0.0.

Recommendation: math.div(4, 3) or calc(4 / 3)

More info and automated migrator: https://sass-lang.com/d/slash-div

  ╷
6 │     width: (4 / 3) * $size;
  │             ^^^^^
  ╵
    node_modules/flagpack/src/_mixins.scss 6:13  fp-size()
    node_modules/flagpack/src/flagpack.scss 8:5  @import
    assets/scss/vendor/flagkit.scss 7:9          @import
    assets/scss/vendor/vendor.scss 7:9           @import
    assets/scss/bolt.scss 3:9                    root stylesheet

: Using / for division outside of calc() is deprecated and will be removed in Dart Sass 2.0.0.

Recommendation: math.div($fp-size, 6) or calc($fp-size / 6)

More info and automated migrator: https://sass-lang.com/d/slash-div

Now npm build runs smoothly again:

Running webpack ...


✔ Webpack
  Compiled successfully in 28.86s

 DONE  Compiled successfully in 28501ms                                                                                  13:32:09

322 files written to public/assets
webpack compiled successfully

Bonus: Fixes #3275