[Enhancement]: Validate inputs for custom network names

[kevinghim]

Background

When there is a case mismatch on a network's currency, i.e. xDai instead of xDAI, we currently show a warning and reject that network as a potential scam. Community input here.

User story

• As a user, I want MetaMask to autofill the network's currency
• If it's not autofilled, provide a warning when the network's currency differs from the source