Allow organizations to change settings via policies

[lieser]

Thunderbird can be customized by organizations via policies, see https://support.mozilla.org/en-US/products/firefox-enterprise/policies-customization-enterprise/policies-overview-enterprise.

Add-ons can add support to be configurable via the same mechanism, see https://extensionworkshop.com/documentation/enterprise/adding-policy-support-to-your-extension/ (at least in Firefox, not yet tested in Thunderbird).

The add-on should allow to set the default values for the non account specific options.
Optionally also allow to lock the settings.

See #267 for a workaround using about:config and autoconfig until this is implemented.

[klou]

I've been working with another developer, and we've worked out a proof of concept that utilizes Managed Storage and can be configured via policies.json, since apparently WebExtension addons will lose access to about:config. This has been tested on TB91.

thunderbird/enterprise-docs#5
https://github.com/thundernest/sample-extensions/tree/master/managedStorage

[lieser]

Thanks for letting me know that the managed storage was already successfully tested.

Note that WebExtension never had direct access to about:config, but instead always required the use of so called experiments to access it. Which hopefully will still work for some time, as the DKIM verifier relies on experiments for some core functionality (see also https://github.com/lieser/dkim_verifier/wiki/FAQ#why-does-the-add-on-need-full-unrestricted-access-to-thunderbird).
So the workaround should still work in TB91.

Btw, please let me know if you or someone else would also like to lock some settings. Otherwise I will probably leave that out then I will start working on this.