thunderbird: Update to 68.7.0

Changelog:
What's New
new MailExtensions: Raw message source available to MailExtensions

Changes
changed MailExtensions: messages.update function extended to mark messages as junk or not junk
changed MailExtensions: browser.compose.begin functions no longer expand mailing lists

Fixes
fixed Various improvements to account setup when connecting to an Exchange server
fixed Thread collapsed when opening news message in a new window
fixed Addons not automatically updated to compatible version after upgrade from Thunderbird 60
fixed Updating addons did not prompt when requesting new permissions
fixed Extra recipients panel not keyboard-accessible
fixed Accessibility: Status bar was not detected by screenreaders
fixed MailExtensions: messages.query by folder name did not require accountsRead permission
fixed Calendar: Invitations with embedded null bytes did not always decode correctly
fixed Calendar: Cancelled events didn't show with a line-through
fixed Various security fixes

Security fixes:
#CVE-2020-6819: Use-after-free while running the nsDocShell destructor
#CVE-2020-6820: Use-after-free when handling a ReadableStream
#CVE-2020-6821: Uninitialized memory could be read when using the WebGL copyTexSubImage method
#CVE-2020-6822: Out of bounds write in GMPDecodeData when processing large images
#CVE-2020-6825: Memory safety bugs fixed in Thunderbird 68.7.0

(ryoon)