Partial revert of b18b3e2: fix bare username

When domain selector is active, username should be a bare node name,
and using `Strophe.getNodeFromJid()` in such cases returns `null`.
Add support for custom modal classes

`Candy.View.Pane.Modal.show()` now takes optional 4th arg
`modalClass` which will be attached to the newly revealed modal
box.

Additionally, all modals get standard class `modal-common`.

Adjust CSS to apply common attributes to `#chat-modal.modal-common`
so that specificity doesn't prevent custom modal classes overriding
attributes.

Lastly, make login modals wider only when domain selector enabled.