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
this manages a gotcha I tried to outline in the description: 

`locale_name` needs to know how to deal with the locale `be`. As it was, the method would return nil and render 
`see this website in ????`  in frontpage locale banner

This checks in to see what language would be falling back to, then names that lanuguage