Update Mailspring to 1.7.1

Summary:
Updated Mailspring to 1.7.1 which includes multiple bug fixes, new translation features, and performance improvements.

Summarized Changelog:

• Mailspring now displays unread mail notifications reliably, fixing a race-condition that occurred when new messages were synced too fast.
• You can now re-order your email acccounts via drag and drop in Preferences > Accounts.
• Mailspring always displays non-inline image attachments as files in the composer. Previously they looked identical to inline images.
• The French, Slovenian, Portuguese, German, Tamil, and Chinese translations have been improved.
• Mailspring does a better job removing quoted text formatted by very old versions of Outlook.
• Mailspring no longer freezes when you type al ink with a "#hash" and then press return.
• On Linux and Windows, the system tray no longer turns blue whenever the app's window is hidden.
• A new option in Preferences > General allows you to disable the shortening of recipient names.
• A new option in Preferences > General allows you to expand message headers by default.
• You can now dismiss Mailspring's upgrade prompt on Linux.
• You can now type date shorthands (eg: "2d" or "2w") into the snooze date picker.

Test Plan: Tested 1.7.1 by sending, recieving, and archiving, emails from a Gmail account. Everything looked and functioned as expected.

Reviewers: Triage Team, JoshStrobl

Reviewed By: Triage Team, JoshStrobl

Subscribers: JoshStrobl

Differential Revision: https://dev.getsol.us/D7412