Remove obsolete service message system commands

[noplanman]

?	!
Type	bug / improvement
BC Break	no
Fixed issues	#994

Summary

Remove all obsolete service message system commands and instead try to execute any user-defined overrides directly in GenericmessageCommand.

Usage

You need to set SystemCommand::$execute_deprecated = true; before handling the request, either in your hook or getUpdates file.

If you have a custom GenericmessageCommand implementation, you'll need to add the code snippet from the original GenericmessageCommand.php to your own.

[jacklul]

I don't think renaming GenericmessageCommand to MessageCommand is a good idea, might get confusing when people create /message commands (no idea about class name conflicts but IDE's might get confused?).

Assuming we get to implement #919 they should be able simply to extend this and parent::execute() (check if empty response) instead of creating new class and copying code, this also prevents people from removing essential code for handling conversations.

[noplanman]

@jacklul I've updated this a bit and moved the code to SystemCommand, to make the integration into a custom GenericmessageCommand easier.

Also, there's a property SystemCommand::$execute_deprecated which needs to be set for this to work. I added this, to not have all the code run for users who don't need this feature.

Any thoughts are highly appreciated! 🙏

I'm also thinking about how best to implement the new command system, to make it logical and easy to use.