i18n fallback fix

this allows
- `be` to fallback to `be-BY`   (belerusian)
- `pt` to fallback to `pt-BR`   (portugeuse)

the solution comprises a few parts:
- adding a stub `fallback.be.yml` so that i18n auto-includes `be` in `available_locales`
- adding a constant to the locales_helper to track fallback locales
- adding switch in the method `locale_name` so that `be` knows what to call itself and doesn't just offer ` see this website in ------ `  in frontpage locale banner

this last one could be done differently, open for feedback
woops, I didn't take advantage of this. I had in mind to call this in the local_helper like 

Loomio::Application.LOCALE_FALLBACKS  

but hadn't linked it up. 
Is that a terrible idea?