Fully prevent the use of path-based IPFS gateways

Currently, we allow users to set whatever IPFS gateway they want except `gateway.ipfs.io`. We should prevent all path-based gateways by implementing the fix suggested below.

_[Original comment](https://github.com/MetaMask/metamask-extension/pull/7362#discussion_r357377132) by @lidel:_

To make this more bullet proof, you could add a quick validation, to ensure provided hostname is a real [subdomain gateway](https://docs.ipfs.io/guides/guides/addressing/#subdomain-gateway), and refuse to save invalid ones.

Try fetching a small, well-known hash (for example: string `hello` is `bafkreicysg23kiwv34eg2d7qweipxwosdo2py4ldv42nbauguluen5v6am`) and see if returned value matches expectations:

https://bafkreicysg23kiwv34eg2d7qweipxwosdo2py4ldv42nbauguluen5v6am.ipfs.dweb.link
https://bafkreicysg23kiwv34eg2d7qweipxwosdo2py4ldv42nbauguluen5v6am.ipfs.cf-ipfs.com
This would also protect you from users setting [path-based gateways](https://docs.ipfs.io/guides/guides/addressing/#path-gateway) (which do not provide Origin-based isolation.)

The issue does not name a file or test. Start at the setting and save path for custom IPFS gateways; validate the hostname as a subdomain gateway and use the provided CID to confirm the expected “hello” response. Done means path-based gateways are rejected while valid subdomain gateways remain usable.

Written by the indexing model from the issue text.