fix #1501 wallet: display right fee amount

[janherich]

fixes #1501

Summary:

Previously, to be able to format fractional number to fixed amount of decimal places, string returned from web3.fromWei call was parsed into native js float type with parseFloat function. Unfortunately for some amounts (not immediately obvious during developer testing), this is not enough, as native js float has problems with precision, so slightly different sum was displayed between fee/amount edits.
This PR solves it by parsing web3.fromWei result directly into BigNumber type (this type should be returned from web3.fromWei call anyway, but it depends on call arguments) and calling toFixed(n) method in it.

Steps to test:

• Open Status
• Open 1-1 Chat
• Start typing /send command and select amount 10
• Push the fee slider to the right (max), remember the fee shown
• Edit the amount to 1
• Edit the amount back to 10
• Compare the fee with the fee shown for amount 10 for the first time (before amount edit), they should be the same

status: ready