Override Torbirdy default settings in a more maintainable way

Originally created by @intrigeri on #10905 (Redmine)

I see that we override Torbirdy default prefs by patching /usr/share/xul-ext/torbirdy/chrome/content/preferences.js with with config/chroot_local-patches/torbirdy-adjust-defaults.diff. It certainly works, and was totally good enough for the initial iterations, but this is the kind of things that’s a pain to maintain: patches get fuzzy, one has to rebase them, and one has to review diffs-of-diffs.

In general, when software offers ways to configure it, let’s use them, instead of patching the source code. The good news is that Torbirdy offers quite some config tools meant for downstream like us :)

E.g.

• We can use /etc/xul-ext/torbirdy.js.
• Upstream provides a way to define various Enigmail torification modes (see setEnigmailPrefs); I think we should add the one we want, instead of patching the source code.
• For the proxy settings, we can either do the same (see e.g. pub.setProxyWhonix), or keep using a custom one; if the latter, it looks like we might be able to set extensions.torbirdy.custom.network.proxy.socks_port in our prefs file, instead of patching the code (untested).

Feature Branch: feature/6154-secure-autoconfig-in-icedove

Parent Task: #5663 (closed)