Adding a new server to the room directory is unreliable in validating the new server

It feels like several things might be going wrong:
* We might be debouncing the text input too aggressively, meaning the validation can lag by several seconds
* We might be getting stuck behind trying to look up nonexistent hosts, causing the validation to lag
* We might not be rechecking the lookup once the user has finished typing
* We might not be cancelling previous lookups as the user types, causing the validation to fall behind

...and apparently we expect a raw servername rather than an https:// URL, which isn't made clear from the placeholder text.

Eitherway, the result is that it's very easy to get a validation fail like this for a hostname which is actually valid: