(Fix) Validate custom RPC

[vbaranov]

Relates to #76

Validate custom RPC with HTTP GET request before adding it to the list.

Also,

• fix styles for address value in the main screen
• add RPC endpoint in parentheses to loader "Connecting to an unknown private network (RPC_ENDPOINT)"

[vbaranov]

@dennis00010011b please add to this PR update of e2e tests to support validation of custom RPC

[fvictorio]

If you add an invalid RPC, an error message appears. If you then change the URL to a valid one and click "Save", the network is saved but the error message is still there.

[vbaranov]

If you add an invalid RPC, an error message appears. If you then change the URL to a valid one and click "Save", the network is saved but the error message is still there.

@fvictorio fixed