Add gas estimation to the postTransaction RPC

[Tbaut]

• Do not add any "gas buffer" (currently 25% for any transaction) when we realize that the transaction is between 2 accounts, and therefore costs invariably 21'000 Wei
• Pass the gas information to the parity_postTransaction RPC to prevent any error from the node. The node knows nothing about the transaction and does not estimate gas (too costly) so it defaults to an amount of gas that is much greater than 21000.
• Should fix Get to the password screen with not enough funds #265 and Prevented from sending transaction when have exactly enough Ether to send an amount and pay total transaction fee #250

[Tbaut]

Should be good to go for ETH transactions, albeit dodgy code, let me know what you think.
I was able to send the exact amount of ETH and leave an account to 0, I tested and didn't break sending tokens so far.
I want to add the gas field in the RPC also for Tokens transfer, so that we avoid weird answers from the node. I'll do this once you have a first look at this code.

[amaury1093]

For future reference, if anyone is reading this PR:
An alternative, cleaner but more complicated solution would be to put gas as a derived form value, using https://github.com/final-form/final-form-calculate, for a future PR.

[Tbaut]

Let me know what you think about that, I could use final-form-calculate, it works as expected for ETH and tokens, but I can't get rid of a warning message: Warning: Form decorators should not change from one render to the next as new values will be ignored

[amaury1093]

It's cleaner imo, but estimateGas() shouldn't be called in validateAmount() anymore.

[Tbaut]

Thanks a lot, I addressed the comments and it should look better now. No more error, token transfer also have gas in postTransaction now.

[amaury1093]

nice! Only some grumbles left

[Tbaut]

Thanks again, comments addressed :)

[amaury1093]

Sorry, found 2 more small grumbles

[Tbaut]

No problem, I think it makes a lot of sense and simplifies things quite a bit. I've tested, as always, and it looks like it works :)

[amaury1093]

🎉

[axelchalon]

👍