(Improvement) Limit user input size

[phahulin]

• What is it? (leave one option)
  • (Improvement) proposal (i.e. some existing feature can be improved)
  • (Refactor) proposal (i.e. no new features or updates, just code optimization/library updates)

https://github.com/poanetwork/poa-popa/blob/e259cec1fcfcfdff30a52bffb395d845c774855b/web-dapp/controllers/prepareRegTx.js#L15

It's best to limit size of user input strings to, say, 500