Rename sb command 'scrollback' and make 'sb' an alias

While this command exists in irssi and might be known by some already,
'sb' is not be the most obvious command name for newcomers.

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

poezio/tabs/basetabs.py

@@ -494,10 +494,11 @@ class ChatTab(Tab):
             usage='<message>',
             shortdesc='Send the message.')
         self.register_command(
-            'sb',
+            'scrollback',
             self.command_scrollback,
             usage="end home clear status goto <+|-linecount>|<linenum>|<timestamp>",
             shortdesc='Scrollback to the given line number, message, or clear the buffer.')
+        self.commands['sb'] = self.commands['scrollback']
         self.register_command(
             'xhtml',
             self.command_xhtml,