Changed bookmark symbol from unicode glyph to 'b'

The glyph previously used '🠷' is interpreted with different
widths on different terminal emulators (i.e. xterm gets it wrong)
This results in visual glitches in the status detail area and
especially in the scrollbar when visible.  Changing it to a
standard ASCII 'b' fixes all these issues.

toot/tui/timeline.py

@@ -373,7 +373,7 @@ class StatusDetails(urwid.Pile):
 
         yield ("pack", urwid.Text([
             ("blue", f"{status.created_at.strftime('%Y-%m-%d %H:%M')} "),
-            ("red" if status.bookmarked else "gray", "🠷 "),
+            ("red" if status.bookmarked else "gray", "b "),
             ("gray", f"⤶ {status.data['replies_count']} "),
             ("yellow" if status.reblogged else "gray", f"♺ {status.data['reblogs_count']} "),
             ("yellow" if status.favourited else "gray", f"★ {status.data['favourites_count']}"),