Custom "API ADDRESS" field communicate what the valid address is

Per https://github.com/ipfs-shipyard/ipfs-webui/issues/1541#issuecomment-662421803 current language regarding custom API address can be misleading because:

1. webui can not dial arbitrary addresses (e.g. https://webui.ipfs.io/ won't be able to dial any http:// address other than loopaback address.
2. Safari can't dial any http address
3. IPFS desktop could probably dial any address
4. Some addresses may not provide CORS headers to allow dialing

I think we can do better by validating inputs and providing contextual information e.g.

- Try to dial and infer why.
- If call was blocked highlight input field and provide message saying it's not dialable.
- If Safari is dialing and it's to localhost we could add little hint explaining that Safari doesn't comply with web standards.
- If non safari is dialing to http other than loopback address, highlight input and provide message explaining mixed content limitation.
- If it's to localhost or HTTPS still got blocked, message mentioning CORS headers