16238 map custom-link button colors to tabler

[arthanson]

Fixes: #16238

Adds CSS styling for missing btn-white, btn-black, btn-grey. Note CSS extend doesn't catch all potential styling on elements depending on how the selectors are defined (see https://stackoverflow.com/questions/13335544/how-to-alias-class-name-in-css-or-sass) but it appears all the styling is good for these buttons.

Note: ghost-dark (used for LINK type) is a tabler button but in the version of tabler we use link text doesn't display in dark mode, should be fixed in latest version of tabler.

[jeremystretch]

Rather than re-mapping the color values, could we just add CSS aliases to provide backward compatibility? IMO this is a less confusing approach.

.btn-white {
  @extend .btn-light;
}

.btn-black {
  @extend .btn-dark;
}