Fix ContentModerator Middleware

[mdrichardson]

Fixes: https://stackoverflow.com/questions/57774564/content-moderator-in-virtual-assistant-template

Purpose

There's no example for how to use ContentModerator and it's a bit difficult to get working right. See above linked issue/answer for more detail.

Note: I'm not terribly familiar with the PR process for VA. Happy to update as necessary.

Changes

1. Removes the Microsoft.Azure.CognitiveServices.ContentModerator NuGet package from the relevant projects. It's been added to Microsoft.Bot.Builder.Solutions and having it in the VA project causes a "Could Not Load Type..." error.
   In /Adapters/DefaultAdapter.cs and DefaultWebsocketAdapter.cs, it adds (near the rest of the middleware pipeline):

// Uncomment and fill in the following line to use ContentModerator
// Use(new ContentModeratorMiddleware(settings.ContentModerator.Key, "<yourCMRegion>"));

Tests

Tested in a bot, but had some difficulty running the VA tests.

Feature Plan

No

Checklist

General

• I have commented my code, particularly in hard-to-understand areas
• [N/A] I have added or updated the appropriate tests
• [N/A] I have updated related documentation

Bots

• [N/A] I have validated that new and updated responses use appropriate Speak and InputHint properties to ensure a high-quality speech-first experience
• [N/A] I have replicated language model changes across the English, French, Italian, German, Spanish, and Chinese .lu files and validated that deployment is successful

Deployment Scripts

• I have replicated my changes in the Virtual Assistant Template and Sample projects
• [N/A?] I have replicated my changes in the Skill Template and Sample projects

[ryanisgrig]

Thanks @mdrichardson, closing #2318 with this