Copy the severity from duplicate bugs

[suhaibmujahid]

Resolves #1705

This PR should not be merged before having the new "Triaged" button deployed on Bugzilla; see #1705 (comment).

Checklist

• Type annotations added to new functions
• Docs added to functions touched in main classes
• Dry-run produced the expected results
• The to-be-announced tag added if this is worth announcing

[suhaibmujahid]

Dry-run

The following bugs have fields that we copied from their duplicate bugs:

Bug	Summary	Copied Fields
1800963	High freq dom/base/crashtests/<test> | assertion count 1 is more than expected 0 assertions	Severity: S4
1800095	Restart button is misaligned in About dialog	Severity: S3
1793807	Hide synced tabs from Firefox View	Severity: S3
1769763	Massive memory use in structured-clone-holder	Severity: S2
1753565	If users set prefers-reduced-motion through system settings, then Firefox should not fire "Smooth Scrolling."	Whiteboard: [access-s3]