Disable unsafe MS Exchange autoconfig method in Thunderbird

Originally created by @anonym on #17654 (Redmine)

While working on #17277 (closed) I noticed that not only had we forgotten to disable the MX autoconfig method, but also the MS Exchange one. However, there seems to be proglems with the latter one, so let’s deal with it in this ticket instead.

Details for the problem can be found here: #17277 (comment 3967)

Feature Branch: bugfix/17654-disable-exchange-autoconfig

Related issues

Blocked by #17277 (closed)

Edited Jun 26, 2020 by intrigeri

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