[META] Addon signing

[andrenatal]

Actionable items:

• Remove the legacy in-tree version from Nightly but make sure we keep the pref so the addon could turn on the preferences panel: https://github.com/mozilla/gecko-dev/blob/master/browser/components/preferences/main.js#L538)
• Bump the version before freezing
• Follow these procedures to prepare the addon for signing: https://github.com/mozilla-extensions/xpi-manifest/tree/master/docs
• ask in #addons-pipeline for a new release off revision xxxxx for firefox-translations when we have a final release version
• Perform addon/security review from the addons team in order to have it signed
• Provide architecture docs: Incorporate proper readme and architecture docs #39
• Provide a more detailed explanation and references to the generic data review policy, and follow the data review addons policy as well: https://extensionworkshop.com/documentation/publish/add-on-policies/