Introduce linting rule for haml-lint to catch wrongly used gettext calls

Recently I made introduced this error (https://gitlab.com/gitlab-org/gitlab-ee/issues/6540):

which boils down to :

# wrong
%span _('Audit Events')
# correct
%span= _('Audit Events')

This should be easily detectable by haml-lint, which supports custom linting rules since April. So we should create a rule for that.

/cc @filipa @reprazent @rymai @DylanGriffith