[meta] Copy metadata from duplicate bugs

[suhaibmujahid]

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1. Automatically copy "access" keyword and "access-sX" whiteboard rating when bugs are duplicated #1706
   area-Metadata improvements area-Severity +2
   
2. Automatically copy Performance Impact field when bugs are duplicated #1757
   +0
3. Automatically set severity when a bug has no severity and another bug with a severity is marked as its duplicate #1705
   area-Noise Reduction +1
4. If a confirmed bug is duplicated to an unconfirmed one, confirm the duplicate target too #1765
   +0
5. Do not copy metadata from duplicate bugs for the same field more than once #1768
   +0
6. Copy the Regressed by field from duplicate bugs #1638
   area-Regressions +1
7. Do not copy metadata to bug with the [no-nag] tag #1855
   +0
8. Webcompat triaged bugs marked as duplicates don't get their webcompat flag transfered #1874
   enhancement +1
9. Refactor the duplicate_copy_metadata tool to be more modular #1892
   +0
10. Copy the see_also field from duplicate bugs #1882
    +0
11. Notify about inconsistency in metadata between a bug and its duplicates #1891
    +0
12. Copy crash signatures from duplicate bugs in duplicate_copy_metadata insted of copy_duplicate_info #2182
    area-Crashes area-Metadata improvements +2