global: add translations folder

[ppanero]

• closes https://github.com/inveniosoftware/cookiecutter-invenio-rdm/issues/241

Folder is empty, the user will execute the extract messages first to initialize the corresponding files if they want custom i18n strings.

Question:

• Should the folder be optional (init user input, like site) then be deleted (e.g. in the post script)

Manual steps for it to work (This will be migrated to invenio-cli)

$ pybabel extract --mapping-file=translations/babel.ini --msgid-bugs-address=info@inveniosoftware.org --copyright-holder=CERN --add-comments=NOTE --output-file=translations/messages.pot --input-dirs=./

$ pybabel init --input-file=translations/messages.pot --output-dir=translations/ --locale=de

$ pybabel compile --use-fuzzy --directory=./translations

To update the catalog run:

$ pybabel update --input-file=translations/messages.pot --output-dir=translations/

Application level translations folder is loaded here.

mkdir ~/.virtualenvs/instance-translations/var/instance/translations
cp -R translations/de ~/.virtualenvs/instance-translations/var/instance/translations