i18n: improve "Welcome" translation in Portuguese (#7068)

* i18n: update gender-neutral language for pt and pt-BR

Instead of using "bem-vindo(a)" (a masculine form of "Welcome"
with a "(a)" in the end to mean "bem-vinda" for the feminine form),
use "boas-vindas", which is a gender-neutral form of "Welcome").

There is already precedent for using "boas-vindas" in the
Brazilian Portuguese localization, in `config/locales/pt-BR.yml`.
European Portuguese dictionary Priberam also registers it as a valid form:
https://www.priberam.pt/dlpo/boas-vindas

* i18n: pt-BR minor orthography fix

The form "a bordo" does not take an accent.
http://oredator.com.br/curso-de-redacao/uncategorized/a-bordo-ou-a-bordo