Add Bulgarian to Tails website weblate

The Bulgarian translators have translated the Tails GUI and want to translate Tails website, but it seems Bulgarian is not enabled on https://translate.tails.net/.

Could we add it? cc @zen

To do

From Enable a new language in Weblate:

• exclude the new language in the production website: tails!1997
• add the new language to $weblate_additional_languages
• add .po files to Git
• update Weblate components
• make sure the new .po files land in tails.git's master branch
• fix doc to add the manual push to canonical
• add reviewers