Custom lists for custom networks

To better support custom networks, the following default lists in MetaMask could be useful to consider:
2. Token lists
3. Token icons
4. Conversion rates API for different chains.

Nice-to-have:
1. Network lists (which networks might we be open to 'suggest'?)
1. Block explorer links
2. Transaction history populated by block explorers that like Etherscan, have an API they offer -- these APIs might be structured completely differently, e.g., with different end-points.
3. Gas APIs for custom networks - add error for 'too high gas price' if user sets higher than high estimated (same as on ethereum mainnet)

cc @jacobcantele useful for the conversations with Pat