Support of POA network

• Adds support for POA Network
• Uses it by default (when installing the extension, not when using unpacked)
• Make it the first item in the list
• Uses the POA explorer for showing accounts and transactions
• Uses POA as a symbol instead of ETH
• Adds support os Sokol network
• Adds a link to Sokol faucet
• Adds a link to POA Sokol explorer for txs and address
• Zero exchange rate for Sokol
• SPOA name of the coin for Sokol
• Adds POA/currency conversion rate for poa networks
• Injects web3 even if it is already exists
• Changes icons and name of extensions
• 1 gwei as gas price when using a POA network