[FEATURE] Remove need to approve facebook messenger app.

[ptrckbp]

Is your feature request related to a problem? Please describe.

Setting up Facebook messenger as a channel is time-consuming when you want anyone to be able to chat with your app (not just testers). You need to create an app and have it approved by the Facebook app approval team, and you have to do this for each environment.

Existing platforms like manychat do this for you. Try it out to see https://manychat.com/product/messenger-marketing

Describe the solution you'd like

1. user signs in with Facebook
2. user accepts permissions to manage their Facebook page
3. user is shown the list of pages she / he administers
4. user picks from the list the pages she / he wants managed by bots
5. user selects bot they want to be managed
6. user types Botpress bot url

Describe alternatives you've considered We can do all steps after 2 without a UI, with us editing a database table of facebook_page_url and botpress_bot_url for our customers.

[davidvitora]

This issue should probably be created in the new https://github.com/botpress/messaging repository.