A test for site_branding tag starts to fail with Django 4.0.
It seems to happen as settings.SITE_BRANDING is _("Horizon") and
a translation marker _() is no longer evaluated during rendering.
As a solution, this commit changes the implementation of
site_branding tag to use "simple_tag" method
as django.template.Library.simple_tag() [1] seems to handle
an i18n-ed string properly.
[1] https://docs.djangoproject.com/en/4.0/howto/custom-template-tags/#simple-tags
Closes-Bug: #1980214
Change-Id: I6fdfffbeef2b405da21289d37722e3f068e27fea
b893bcdee3