feat(Contracts): DApp registry contract using ENS and FDS registrar as base

[molekilla]

Refactors to use ENS ERC721 token curated registry on top of FDS registrar.

• Contains merged PR from fds registrar branch
• Refactored DAppregistry.sol
• Unit tests

Not implemented in this release/issue: deployment or migration scripts

[nugaon]

please don't include other PR under review. please rebase it to the other branch and set this PR's base to that branch until it gets merged.

[molekilla]

@nugaon ready for review

[molekilla]

@nugaon will update this PR later this week