Remove empty space when the "Add" button is not shown in the sidebar
Remove empty space when the "Add" button is not shown in the sidebar

The "Add" button is shown only for logged in users, so it is not shown
in guest pages. However, when the chat view is shown in the sidebar in a
guest page there was an empty space where the "Add" button would have
been shown for a logged in user. Now the message input stretchs to use
that empty space.

Note that this is only applied when the chat view is in the sidebar;
when the chat view is in the main view the "Add" button is added in the
padding area used to frame the chat view, so in that case the empty
space should be kept.

Signed-off-by: Daniel Calviño Sánchez <bd7629e975cc0e00ea7cda4ab6a120f2ee42ac1b@gmail.com>

nextcloud/spreed,nextcloud/spreed,nextcloud/spreed