update bootstrap 4.3.1, update gulpfile, npm start script added, change SCSS file names for proper compiling

vendor/bootstrap/scss/mixins/_deprecate.scss

@@ -0,0 +1,10 @@
+// Deprecate mixin
+//
+// This mixin can be used to deprecate mixins or functions.
+// `$enable-deprecation-messages` is a global variable, `$ignore-warning` is a variable that can be passed to
+// some deprecated mixins to suppress the warning (for example if the mixin is still be used in the current version of Bootstrap)
+@mixin deprecate($name, $deprecate-version, $remove-version, $ignore-warning: false) {
+  @if ($enable-deprecation-messages != false and $ignore-warning != true) {
+    @warn "#{$name} has been deprecated as of #{$deprecate-version}. It will be removed entirely in #{$remove-version}.";
+  }
+}