Rename the translation platform maintainers list

Currently, the translation platform maintainers team use a list called tails-translations@. The name of that list causes confusions, because:

• people end up sending things that should instead go to tails-l10n@, and
• the translation platform maintainers doesn't intend to take over the scope or diminish the autonomy of tails-l10n@.

Some ideas for a new name:

• tails-translation-platform@
• tails-t9n-platform@
• tails-platform@
• tails-weblate@

Rough steps:

• Create new list, subscribe team.
• Update puppet code. MR: puppet-tails!21 (merged)
• Update the documentation. MR: tails!200 (comment 157993)
• Update email alerts
• Upload tails-weblate.key to the webserver

@emmapeel, any thoughts on this?