Bugzilla plugin is now converted to backend sync service

as the summary, Bugzilla is now a backend sync service. It can be used simply as it was a plugin before. When enable this service, any added task will be handled and attempt to sync bug information from specific related Bugzilla service.

This version works well, but not perfect. I'm being continue to add more features, such as the configuration ability.
A major thing for this file: it feels too much as Java and less as python (why do you need to create Factory class with a single static function when you can create just a function)

There seems to be absolutely no added value in having a separate object per bugzilla other than checking if the bug comes from a supported bugzilla. Can you remove this whole file?