Allow meta tags to be set from localisation files

Car campaigns landing page now loads meta tags from its localisation
file. #2442
I think we're trying to be too clever. Our only requirement as this time is to support a description. We should just make the simple change.

```erb
<% set_meta_tags(description: t('.description'),
                 canonical:   campaigns_revealed_the_true_cost_of_buying_a_car_url(locale: :en),
                 alternate:   { en: campaigns_revealed_the_true_cost_of_buying_a_car_url(locale: :en),
                                cy: campaigns_edrychwch_cost_gwirioneddol_prynu_car_url(locale: :cy) } %>
```