[l10n] String extraction duplicates shared strings within addons

[flodolo]

@mcleinman

Unfortunately the string extraction is not working as it should: shared strings (like vpn.commonStrings.generalUpdateBulletIntro), are extracted twice, once in en/addons/strings.xliff, once in the addon (e.g. en/addons/message_update_v2.25/strings.xliff).

Should we ignore strings starting with vpn.commonStrings when generating addons? While it could be done in the l10n repository as an additional workflow step, I think the clean approach is to fix it in this repo, where addon strings are extracted.

In order to unblock translation, should I go ahead and manually remove shared strings from whats_new_v2.25?

[mcleinman]

Yes, we can manually remove shared strings from whats_new_v2.25 and message_update_v2.25, or just close that current PR.

I have a fix for this up, and the description includes more diagnosis / explanation. Sorry about this.