Migrate from Enigmail to Thunderbird 78's built-in OpenPGP support

The next Thunderbird ESR after v68 will have its own OpenPGP support and Enigmail will go away:

• https://blog.mozilla.org/thunderbird/2019/10/thunderbird-enigmail-and-openpgp/
• https://wiki.mozilla.org/Thunderbird:OpenPGP:2020

Parent Task: #17148

Related issues

• Blocks #16209
• Blocks #16477

Original created by @intrigeri on 17147 (Redmine)