[mcsvr] Changing mode does not work

[Deleted user]

See #26 for a potential fix.

Cog: mcsvr

Description of the problem:
When I try to run [p]mcset mode embed yes to set the tracker mode to embed I get the following error on my console:

[31/12/2018 08:20] ERROR events on_command_error 208: Exception in command 'mcset mode'
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/discord/ext/commands/core.py", line 61, in wrapped
    ret = await coro(*args, **kwargs)
  File "/home/alice/red-discordbot/data/cogs/CogManager/cogs/mcsvr/mcsvr.py", line 194, in mcset_mode
    await self.do_mode_toggle_cleanup(current_mode, ctx.guild)
  File "/home/alice/red-discordbot/data/cogs/CogManager/cogs/mcsvr/mcsvr.py", line 216, in do_mode_toggle_cleanup
    await self.config.channel(channel).current_server.set("")
  File "/usr/local/lib/python3.6/dist-packages/redbot/core/config.py", line 241, in __getattr__
    raise AttributeError("'{}' is not a valid registered Group or value.".format(item))
AttributeError: 'current_server' is not a valid registered Group or value.
[31/12/2018 08:20] ERROR events on_command_error 213: Exception in command 'mcset mode'
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/discord/ext/commands/core.py", line 61, in wrapped
    ret = await coro(*args, **kwargs)
  File "/home/alice/red-discordbot/data/cogs/CogManager/cogs/mcsvr/mcsvr.py", line 194, in mcset_mode
    await self.do_mode_toggle_cleanup(current_mode, ctx.guild)
  File "/home/alice/red-discordbot/data/cogs/CogManager/cogs/mcsvr/mcsvr.py", line 216, in do_mode_toggle_cleanup
    await self.config.channel(channel).current_server.set("")
  File "/usr/local/lib/python3.6/dist-packages/redbot/core/config.py", line 241, in __getattr__
    raise AttributeError("'{}' is not a valid registered Group or value.".format(item))
AttributeError: 'current_server' is not a valid registered Group or value.