Add contract codehash verification to `Add exisiting contract`

[agryaznov]

Currently it is possible to imperceptibly forge contract, i.e. add existing contract with codeHash other than that one specified in ABI

Context

We have two contracts:

1. normal candle_ok.contract (src)
2. malicious modified candle_mal.contract (src)

(both contract builds are attached to this issue)

Steps to reproduce:

1. Upload & Instantiate malicious contract
2. (better in other browser session) Click on Add existing contract, upload ABI of normal contract, but specify the address of the malicious contract instance

Preferred behavior:
Compare the codeHash of contract at specified address with the one specified in ABI, and since it differs, return an error

Actual behavior:
UI shows everything is fine