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 :
```haml
# wrong
%span _('Audit Events')
# correct
%span= _('Audit Events')
```
This should be easily detectable by haml-lint, which supports custom linting rules [since April](https://github.com/brigade/haml-lint/pull/268). So we should create a rule for that.
/cc @filipa @reprazent @rymai @DylanGriffith
issue