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
the fallbacks are used both here and in the `locale_helper`  - they need to be included in the list of 'supported locales' and I thought it was wise to have them seperate to the 'experimental locales' because they are not experimental. 

I think they need to be stored in this file, because of how Rails loads its files. 
My solution was to make a constant so I could access these fallbacks