Add `active_when` helper

We have the following pattern all over the codebase in views:

%li{ class: (klass == Gitlab::GitLogger ? 'active' : '') }

It'd be awesome if we could just do this, for example:

%li{ class: active_when(klass == Gitlab::GitLogger) }

The implementation couldn't be simpler:

def active_when(condition)
  'active' if condition
end