(Feature) Add ERC-725/ERC-735 implementation

[unjapones]

Closes #83 .

• Adds a page (and endpoint in the backend) to generate erc735 claim based on verified bank accounts.
• The page also has a button that allows the addition of the generated claimd to the specified erc725 identity contract.

The following GIF shows the UI interaction:

The branch unjapones/poa-poba/tree/add-erc735-generation-page_with-user-identity-contract that allows the full testing of the code as it is done in the GIF above and in this screencast: https://drive.google.com/file/d/1CTRd516zVOvl9wi2gA8zoZbFM1Tl_zkW/view?usp=sharing

Notes

This PR is similar to another one found in poa-popa, with some considerations

• PoBA uses a more updated version of Web3 (the code that generates the signature in the backend is more straight forward).
• In this PR, PoBA's the identity contract is the main PoBA.sol contract (there was an issue in poa-popa that didn't allowed this.)

[coveralls]

Pull Request Test Coverage Report for Build 238

• 144 of 181 (79.56%) changed or added relevant lines in 14 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-2.8%) to 80.344%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
frontend/src/App.js	0	1	0.0%
frontend/src/ui/containers/PlaidBankAccounts.js	0	1	0.0%
frontend/src/PoBAServer.js	1	3	33.33%
frontend/src/ui/containers/AddClaimToIdentityContractButton.js	19	21	90.48%
frontend/src/ui/containers/GenerateErc735ClaimForm.js	27	29	93.1%
frontend/src/ui/pages/AddClaimToIdentityPage.js	15	18	83.33%
frontend/src/ui/utils/erc725.js	6	10	60.0%
blockchain/contracts/KeyHolder.sol	28	50	56.0%
<!--	Total:	144	181	79.56%	-->

Files with Coverage Reduction	New Missed Lines	%
frontend/src/PoBAServer.js	1	21.43%
<!--	Total:	1		-->

Totals
Change from base Build 237:	-2.8%
Covered Lines:	469
Relevant Lines:	573

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💛 - Coveralls