Set default gas limit to minimum allowed

A transaction initiated from a dapp might not set a gas limit, which
would result in a default of zero being used in the advanced tab. The
default gas limit in that case has been changed to 21,000, the minimum
allowed gas limit, so that users aren't forced to manually update it.

ui/app/components/app/gas-customization/gas-modal-page-container/gas-modal-page-container.container.js

@@ -84,7 +84,7 @@ const mapStateToProps = (state, ownProps) => {
 
   const { gasPrice: currentGasPrice, gas: currentGasLimit, value } = getTxParams(state, selectedTransaction)
   const customModalGasPriceInHex = getCustomGasPrice(state) || currentGasPrice
-  const customModalGasLimitInHex = getCustomGasLimit(state) || currentGasLimit
+  const customModalGasLimitInHex = getCustomGasLimit(state) || currentGasLimit || '0x5208'
   const customGasTotal = calcGasTotal(customModalGasLimitInHex, customModalGasPriceInHex)
 
   const gasButtonInfo = getRenderableBasicEstimateData(state, customModalGasLimitInHex)