[#6612] Allow direct user-mapping for SF Trac imports

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>
[#6612] Preserve original ticket owner if username lookup fails

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>
[#6612] Handle missing fields in imported json

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>