BREAKING CHANGE: Move Talis customisations to own partial

[danielmatthew]

Per Bootstrap's documentation on customisation, this PR specifies a _theme partial, meaning it can sit on top of Bootstrap v5.

Option A: Include all of Bootstrap (185kb)

// custom.scss


// Include any default variable overrides here (though functions won't be available)
@import "bootstrap-theme/scss/variables";
// Assumes Sass load-path includes node_modules
@import "bootstrap/scss/bootstrap"; 

// Then add additional custom code here
@import "bootstrap-theme/scss/theme";

Option B: Include only parts of Bootstrap we use (163kb)

// custom.scss


// 1. Include functions first (so you can manipulate colors, SVGs, calc, etc)
// Assumes Sass load-path includes node_modules
@import "bootstrap/scss/functions";

// 2. Include any default variable overrides here
@import "bootstrap-theme/scss/variables";

// 3. Include remainder of required Bootstrap stylesheets
@import "bootstrap/scss/variables";
@import "bootstrap/scss/mixins";
@import "bootstrap/scss/utilities";

// 4. Include any optional Bootstrap components as you like
@import "bootstrap/scss/root"; 
@import "bootstrap/scss/reboot";
@import "bootstrap/scss/type";
@import "bootstrap/scss/images";
@import "bootstrap/scss/containers";
@import "bootstrap/scss/grid";
@import "bootstrap/scss/tables";
@import "bootstrap/scss/forms";
@import "bootstrap/scss/buttons";
@import "bootstrap/scss/transitions";
@import "bootstrap/scss/dropdown";
@import "bootstrap/scss/button-group";
@import "bootstrap/scss/nav";
@import "bootstrap/scss/navbar";
@import "bootstrap/scss/accordion";
@import "bootstrap/scss/breadcrumb";
@import "bootstrap/scss/pagination";
@import "bootstrap/scss/badge";
@import "bootstrap/scss/alert";
@import "bootstrap/scss/progress";
@import "bootstrap/scss/close";
@import "bootstrap/scss/toasts";
@import "bootstrap/scss/modal";

// Helpers
@import "bootstrap/scss/helpers";

// Utilities
@import "bootstrap/scss/utilities/api";

// 5. Add additional custom code here
@import "bootstrap-theme/scss/theme";