AssetIcon component

[louisinger]

This PR cherry-pick AssetIcon commit from #345

• use AssetIcon to load asset's icon and do not re-assign img source if error occurs (launching a loop causing image blinking).
• use React.memo to do not re-mount asset component each time we render the page (and so do no re-send img requests). Also use React.memo in any others pure functional components (TxIcon, Balance etc...). Why react.memo

@tiero @bordalix please review

[bordalix]

@louisinger to fix the blinking image, we just needed to mv unknown.png unknown.svg (it was a svg file with the wrong extension).

But I see you did more than that here. Makes sense what you did.