scripts: Add render_symbols to test symbol rendering.

This commit adds a new folder `scripts/` to ship scripts that are meant
for users. This commit also adds a new script that enables users to check
how the various symbols listed in `config/symbols.py` render on their
terminal emulator.

The need for this script has arisen from the observed issues on the Windows
Terminal, where with varying fonts, symbol rendering has also varied.

setup.py

@@ -101,6 +101,8 @@ def long_description():
     entry_points={
         'console_scripts': [
             'zulip-term = zulipterminal.cli.run:main',
+            'zulip-term-check-symbols = '
+            'zulipterminal.scripts.render_symbols:main',
         ],
     },
     extras_require={

zulipterminal/scripts/render_symbols.py

@@ -0,0 +1,51 @@
+import sys
+
+import urwid
+from urwid import set_encoding
+
+from zulipterminal.config import symbols
+
+
+set_encoding('utf-8')
+
+palette = [
+    ('header', 'white, bold', 'light blue'),
+    ('footer', 'black', 'white'),
+]
+
+variables = vars(symbols)
+symbol_dict = {symbol: variables[symbol] for symbol in variables
+               if not symbol.startswith('__') and not symbol.endswith('__')}
+
+symbol_names_list = [urwid.Text(name, align='center')
+                     for name in symbol_dict.keys()]
+symbols_list = [urwid.Text(symbol) for symbol in symbol_dict.values()]
+
+# given_width is hardcoded based on the longest symbol name (26 characters).
+# TODO: Update the width if we have a longer name for a symbol.
+symbols_display_box = urwid.Columns([
+    (27, urwid.Pile(symbol_names_list)),
+    (3, urwid.Pile(symbols_list))
+], dividechars=9)
+
+line_box = urwid.LineBox(symbols_display_box,
+                         title="Render Symbols",
+                         title_attr='header')
+info_box = urwid.Text(('footer', u" Exit: ^C "), align='center')
+display_box = urwid.Pile([
+    line_box,
+    info_box
+])
+
+# Allot a width of 45 to the display_box. line_box occupies about 39 characters
+# so we consider that as a maximum. Use extra width to make it look cleaner.
+body = urwid.Filler(urwid.Padding(display_box, align='center', width=45),
+                    valign='middle')
+
+
+def main() -> None:
+    try:
+        loop = urwid.MainLoop(body, palette)
+        loop.run()
+    finally:
+        sys.exit(0)