Simplify setting http response after activity processing

[singhk97]

Linked PR: microsoft/teams.ts#398

This is how the architecture looked like before

This is what it looks like now:

[heyitsaamir]

Now that we aren't using events for responses, I think we should make these fundamental changes:

1. Remove process_activity from event manager and put it back in app. There is no reason for event manager to call process activity now, since the response isn't coming through event manager anymore.
2. So pull the above in app. Inject that function in. That function can call event_manager.on_activity if it wants, but that would be a fire and forget event.
3. The on_activity_response can also be called from the above handler then (instead of from process_message). That would clean up that architecture.

[singhk97]

Refactored the activity processor out of the event manager and into the plugin processor.

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[singhk97]

So it turns out that the error event is not even being emitted if an exception is raised by one of the handlers. I've pushed a commit to a fix that. The behavior is now:

for

(apologies for confusing error message)

[heyitsaamir]

Tested