New variable `%recipient%` for final recipient to replace `%to%` / `%from%` in all templates

[RealRaven2000]

To simplify usage of external HTML templates across the cases Write / Reply / Forward I would like a new variable that intelligently switches between the to and from headers so that it can be used like in the following example

<p>Dear %tofinal(name)%,</p>
<p>%cursor%</p>
<p>yours sincerely,<br>
&nbsp;&nbsp;%identity(firstname)%</p>

this could be used both for writing / replying / forward without having to manually modify the result in composer.

[RealRaven2000]

I found %tofinal% a little clunky, changing to %recipient% which is more distinct, and hopefully makes a nice match with the opposing %identity% (which is a placeholder for the sender composing the email)

[RealRaven2000]

Implemented in version 3.8 - please note that for Thunderbird 68 you have to install new versions from the github releases page.

Unfortunately thunderbird.net does not accept updates for legacy versions anymore - they only support the new "Mail extension" format and our "mail extension with experimental APIs" which is a workaround for missing advanced features.