feat(ledger-browser): handle ERC721 token metadata

[outSH]

Is your feature request related to a problem? Please describe.

Ethereum App on ledger browser should display information stored in ERC721 token metadata file (if available).

Describe the solution you'd like

This is a multi-step issue that require changes in 2 components (sub tasks will be created if issue is assigned).

• Modify cactus-plugin-persistence-ethereum to get metadata URL from tokenURI() and parse the JSON file. It should handle connection errors, invalid and non-conformant files. Fetched data (name, description, image-url) should be pushed to the database. Scenario and error cases should be covered by an automatic test.
• Modify ledger-browser Ethereum Browser App to display new data on ERC721 view in Accounts tab.

Additional context

• Assume JSON metadata matches official ""ERC721 Metadata JSON Schema" as described in https://github.com/ethereum/ercs/blob/master/ERCS/erc-721.md