Use OAuth2 flow for grabbing meetup credentials

[alysivji]

As a developer, I want to use the OAuth2 authentication flow for Meetup. We are currently using the API credential method which is not supported anymore.

Will need to modify the meetup adapter to create an Authorization header

https://www.meetup.com/meetup_api/auth/#oauth2

Task List

• [x] update meetup adapter and tests -- #199
  • [x] remove meetup-api package
• [ ] add meetup oauth provider and endpoint blueprint
  • [ ] add model

[alysivji]

Meetup doesn't accept API keys anymore, so this needs to get done ASAP.