Update login username placeholder to only request "user" instead of "email"

[binarydad]

As mentioned in #53, the full user@domain is no longer required, so it seems fitting that the field placeholder also matches.

[nioc]

As mentioned in my last comment, user can either login with just local jid (using default domain) or full jid.
Some users requested a feature for setting custom websocket transport url.
I think it is easier to understand if placeholder mention xmpp domain.

What I can do is to set the domain in placeholder with the default domain instead of hard-coded one.