Persistent Thunderbird blocks future 0000tails.js prefs changes

Let’s move config/chroot_local-includes/etc/skel/.thunderbird/profile.default/preferences/0000tails.js to /etc/thunderbird/profile/preferences/0000tails.js and make the former a symlink to the latter so any changes to this file will apply to persistent Thunderbird profiles.

Feature Branch: feature/12639-thunderbird-52

Related issues

Related to #12639 (closed)
Related to #15746 (closed)
Blocks #13234 (closed)

Original created by @anonym on 12680 (Redmine)

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