boxes: Pack recipient headers to ensure they are displayed.

This commit allows recipient headers to wrap onto multiple lines, rather
than being a fixed width, which previously caused them to not be
rendered in narrow windows.

This at least shows the recipient header if they are long, however the
styling could be improved. A custom urwid widget may be necessary to
improve further.

Fixes #671.

zulipterminal/ui_tools/boxes.py

@@ -291,7 +291,7 @@ def stream_header(self) -> Any:
         ])
         stream_title = urwid.Text(stream_title_markup)
         header = urwid.Columns([
-            (len(stream_title.text), stream_title),
+            ('pack', stream_title),
             urwid.AttrWrap(urwid.Divider('━'), color),
         ])
         header.markup = stream_title_markup
@@ -304,7 +304,7 @@ def private_header(self) -> Any:
         ])
         title = urwid.Text(title_markup)
         header = urwid.Columns([
-            (len(title.text), title),
+            ('pack', title),
             (1, urwid.Text(' ')),
             urwid.AttrWrap(urwid.Divider('━'), 'general_bar'),
         ])