Add support for read-only accounts

We should add support for "read only" accounts, for users who want to view their account details, and even browse dapps "from that account's perspective", without carrying any authority over that account.

I thought we had this issue open, but I couldn't find it after [it was requested on twitter by Tim at Truffle](https://twitter.com/timothyjcoulter/status/1165821363484942336).

Should be easy to implement, since [the KeyringController protocol](https://github.com/MetaMask/keyringcontroller) makes it easy to add new key types, and this one has literally no logic to it, it's almost a wonder we don't support it already.

The form for adding one could be similar to private key import, except no private key is required. The keyring would just throw an error for any signing operation, or not implement them (the keyring controller should throw the method-not-found errors).

Start by reading the KeyringController protocol and the existing private-key import form and account flow. Define how a read-only account is added and exposed for browsing dapps, then verify that signing operations are rejected with the specified method-not-found behavior. Done means the account can be viewed and used as a dapp perspective without signing authority.

Written by the indexing model from the issue text.