* nft: L-13 Duplicated code

* nft: M-08 Upgradeability inconsistencies

* fix(ctp): change `CrossDomainEnabled` to `CrossDomainMessenger`

* fix(ctp): make `otherBridge` and `messenger` in nft bridge contracts immutable

* docs(ctp): update `ERC721Bridge` bridging function docs

* re-order `OptimismMintableERC721Factory` event args

* remove unnecessary `Initializable` import in nft bridges

* changed legacy `CrossDomainEnabled` call to `CrossDomainMessenger
Co-authored-by: Mark Tyneway <mark.tyneway@gmail.com>