nft: L-13 and M-08

[sam-goldman]

L-13: Refactors a lot of the functionality in the NFT bridges into a single base contract.

M-08: Fixes upgradeability inconsistencies in the ERC721 bridge contracts.

The third commit replaces CrossDomainEnabled with CrossDomainMessenger in the ERC721 bridge contracts because CrossDomainEnabled is not upgradeable.

The fourth commit makes otherBridge and messenger in the NFT bridge contracts immutable as requested here.

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[mergify]

Hey @sam-goldman! This PR has merge conflicts. Please fix them before continuing review.

[mergify]

Hey @sam-goldman! This PR has merge conflicts. Please fix them before continuing review.

[sam-goldman]

Added a commit that re-orders the OptimismMintableERC721Created event as suggested here

[mergify]

Hey @sam-goldman! This PR has merge conflicts. Please fix them before continuing review.

[tynes]

This needs a new rebase

[mergify]

This PR has been added to the merge queue, and will be merged soon.

[mergify]

This PR is next in line to be merged, and will be merged as soon as checks pass.