add dialect support

@robguthrie here's a starter.
We need all locales to use either - or _ .  If we use -, then all the locales will be ugly symbol-strings `:'zh-TW'` ... I think the better option might be to add something which converts all the locale strings nabbed by http_accept_language to the underscore version.

Also want to have a brief chat in person about strings versus symbols in the locale context

confirming the fallbacks:
```ruby
[9] pry(main)> I18n.fallbacks[:pt]
=> [:pt, :pt_BR, :en]
[10] pry(main)> I18n.t('about_loomio', locale: :pt)
=> "Sobre o Loomio"
[11] pry(main)> I18n.t('about_loomio', locale: :pt_BR)
=> "Sobre o Loomio"
```