Entering homeserver URLs is hard

"On Riot for iOS, if you're trying to edit an invalid homeserver URL (like, for example if you typed in chat.mozilla.org first) every second and a half or so a modal popup reminds you that your homeserver URL is invalid, and resets your cursor location... making it difficult to get it fixed. (I ended up retyping from the beginning)"

from `@mgaudet:mozilla.org`

Reproduce the invalid-homeserver editing flow on Riot for iOS, using the reported periodic modal and cursor reset as the starting behavior. Locate the URL validation and editing screen from that flow; done means an invalid URL can be corrected without repeated interruption or cursor repositioning, with a regression test if the project provides one.

Written by the indexing model from the issue text.