Thunderbird 60.5.0

What's new in this version:

New:
- FileLink provider WeTransfer to upload large attachments
- Thunderbird now allows the addition of OpenSearch search engines from a local XML file using a minimal user inferface: [+] button to select a file an add, [-] to remove
- More search engines: Google and DuckDuckGo available by default in some locales
- During account creation, Thunderbird will now detect servers using the Microsoft Exchange protocol. It will offer the installation of a 3rd party add-on (Owl) which supports that protocol

Fixed:
- Thunderbird now compatible with other WebExtension-based FileLink add-ons like the Dropbox add-on
- Crash when using custom sound for new email notification
- Various security fixes