Improve permission_explanation for prematurely closed Periods

- Periods closed improperly through the 'close' command were not being
shown properly through the 'status' command.
- Fixed error in Submission state checking in on_message handling.
- Added embed support to 'close' command message responses.

bot/cogs/contest_commands.py

@@ -210,13 +210,13 @@ class ContestCommandsCog(commands.Cog, name='Contest'):
             period: Period = guild.current_period
 
             if period is None or not period.active:
-                await ctx.send('No period is currently active.')
+                await ctx.send(embed=helpers.error_embed(message='No period is currently active.'))
             else:
                 overwrite = discord.PermissionOverwrite()
                 overwrite.send_messages = False
                 overwrite.add_reactions = False
                 period.deactivate()
-                await ctx.send('The current period has been closed.')
+                await ctx.send(embed=helpers.success_embed(message='The current period has been closed.'))
 
     @commands.command()
     @commands.guild_only()