[FIX] awesome_owl: Fix missing css variables

Some imports are missing in the manifest causing some warnings and
the css doesn't load properly.
Fix was fixed in the master branch, backporting it in 19.0
for the onboarding classes that always happen in the lastest stable.

task-none

Part-of: #1037
Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>

Author: clbr-odoo

awesome_owl/__manifest__.py

@@ -29,8 +29,10 @@
     'assets': {
         'awesome_owl.assets_playground': [
             ('include', 'web._assets_helpers'),
+            ('include', 'web._assets_backend_helpers'),
             'web/static/src/scss/pre_variables.scss',
             'web/static/lib/bootstrap/scss/_variables.scss',
+            'web/static/lib/bootstrap/scss/_maps.scss',
             ('include', 'web._assets_bootstrap'),
             ('include', 'web._assets_core'),
             'web/static/src/libs/fontawesome/css/font-awesome.css',