Merge Thunderbird add-ons website with addons.mozilla.org

[Ponda]

I think this is straightforward request. Benefits are obvious, yet I will list some:

* current Thunderbird add-ons is very outdated

* helps to spread awareness of Thunderbird among Firefox users and add-on devs

* enables the possibility for creating themes compatible with both Firefox and Thunderbird

* makes it easier to publish Thunderbird extensions based on existing Firefox add-ons.

This would also help to enable Thunderbird support in Firefox Color (see https://connect.mozilla.org/t5/ideas/update-and-improve-firefox-color/idi-p/62017 for details).

[Jon]

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