Bug 42356: Silence Sass if-function deprecation warning

Like Bug 41327, deprecation warnings about Bootstrap SCSS should be
silenced with the assumption that a future version of Bootstrap will
correct the problems. In this case the warning is:

Deprecation Warning [if-function]: The Sass if() syntax is deprecated in
favor of the modern CSS syntax.

To test, apply the patch and run 'yarn build.' There should be no
warnings.

Sponsored-by: Athens County Public Libraries
Signed-off-by: David Nind <david@davidnind.com>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>

gulpfile.js

@@ -35,6 +35,7 @@ var sassOptions = {
     silenceDeprecations: [
         "color-functions",
         "global-builtin",
+        "if-function",
         "import",
         "legacy-js-api",
     ],