The submit button in the chat view is positioned using its "bottom"
property to align it with the bottom of the new message form even when
there are several lines and the form grows vertically.
"publicshareauth.css" (core) sets the "top" property of submit buttons,
which causes the submit button in the chat view to be aligned to the top
of the new message form, and thus needs to be overriden.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

While a message is being sent the submit button in the chat view is
hidden by setting the "hidden" CSS class to it. However, in "guest.css"
an explicit "display" property is set for submit buttons; this rule is
more specific than the generic rule for ".hidden" elements, and thus
overrides it, causing the submit button to be shown even when the
"hidden" CSS class is set. Therefore, an explicit rule is needed to set
"display: none" for hidden submit buttons.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>