Botbuilder-samples: Refactor sample 57

[clearab]

The messageAllMembers functionality in sample 57 needs to be refactored. As written, it is encouraging developers to call createConversation repeatedly, and is difficult to read. We need to separate out the create and continue calls, and use in-memory storage to save the conversation reference.

[clearab]

It may make more sense to pull the proactive portion out of this sample entirely, and refactor sample 58 into a proactive messaging sample all-up (sending proactive to the various constructs, using a something outside of the turnContext to send the messages to make it more clear).

[clearab]

For additional context, see the conversation here: https://gitter.im/OfficeDev/MicrosoftTeamsAppDev?utm_source=notification&utm_medium=email&utm_campaign=unread-notifications#

[HiltonGiesenow]

@clearab thanks for creating this issue - I'm more than happy to help out on this.

[clearab]

@EricDahlvang let's talk about this one next week