Update thunderbird to 68.2.0

Summary:
New:

• Message Display WebExtension API
• Message Search WebExtension API

Fixed:

• Better visual feedback for unread messages when using the dark theme
• Various issues when editing mailing lists
• Application windows not maintaining their size after restart
• Visual glitches: Missing context menu in filter, downloads, password manager and Config Editor search boxes, unwanted scrollbars and cut-off text in Account Manager, incorrect colors in Calendar agenda scrollbars, theme issues on Windows 7
• Some attachments couldn't be opened in messages originating from MS Outlook 2016
• Address book import form CSV
• Performance problem in message body search
• Ctrl+Enter to send a message would open an attachment if the attachment pane had focus
• Calendar: Issues with "Today Pane" start-up
• Calendar: Glitches with custom repeat and reminder number input
• Calendar: Problems with WCAP provider

Signed-off-by: Pierre-Yves <pyu@riseup.net>

Test Plan: Check my mails

Reviewers: Triage Team, JoshStrobl

Reviewed By: Triage Team, JoshStrobl

Subscribers: JoshStrobl

Differential Revision: https://dev.getsol.us/D7476