Add discord plugin

[ImDevinC]

Checks

• I tested my changes or release
• I agree to the terms of service

[Core447]

Some things I noticed:

1. The thumbnail doesn't seem to load in the store. Can you maybe try to change it to a png with transparent layer?
2. When I add a "Mute" action directly after the installation (so without any config), I get this:

File "/app/bin/StreamController/src/backend/DeckManagement/DeckController.py", line 1435, in own_actions_update
    action.on_update()
  File "/app/bin/StreamController/src/backend/PluginManager/ActionBase.py", line 129, in on_update
    self.on_ready() # backward compatibility
    ^^^^^^^^^^^^^^^
  File "/home/core447/Documents/StreamController-config/plugins/com_imdevinc_StreamControllerDiscordPlugin/actions/MuteAction.py", line 20, in on_ready
    self.plugin_base.backend.register_callback(
  File "/app/lib/python3.11/site-packages/rpyc/core/netref.py", line 239, in __call__
    return syncreq(_self, consts.HANDLE_CALL, args, kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.11/site-packages/rpyc/core/netref.py", line 63, in syncreq
    return conn.sync_request(handler, proxy, *args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.11/site-packages/rpyc/core/protocol.py", line 744, in sync_request
    return _async_res.value
           ^^^^^^^^^^^^^^^^
  File "/app/lib/python3.11/site-packages/rpyc/core/async_.py", line 111, in value
    raise self._obj
_get_exception_class.<locals>.Derived: 'NoneType' object has no attribute 'subscribe'

========= Remote Traceback (1) =========
Traceback (most recent call last):
  File "/app/lib/python3.11/site-packages/rpyc/core/protocol.py", line 369, in _dispatch_request
    res = self._HANDLERS[handler](self, *args)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.11/site-packages/rpyc/core/protocol.py", line 863, in _handle_call
    return obj(*args, **dict(kwargs))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/core447/Documents/StreamController-config/plugins/com_imdevinc_StreamControllerDiscordPlugin/backend.py", line 62, in register_callback
    self.discord_client.subscribe(key)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'subscribe'

3. It would be nice to also show the failed to setup discord client: Error, could not find Discord. is Discord running? error in the ui

[ImDevinC]

@Core447 thanks for the validation. Updated accordingly in this latest release

[Core447]

I still have the problem with the missing thumbnail. This PR should fix this.

[ImDevinC]

Updated, thanks for the PR!