Support multi driver

[shaunagm]

Very early stages of PR for adding multi-driver support, which enables us to create the hosted gateway. Nothing has been tested.

Steps:

• create basic database models (no migration files yet, these are initial models that may need to change)
• get_config method in core.utils and httpwrapper.utils, along with logic to determine which to use
• update twitter plugin to make api calls with get_config
• update github plugin to do token exchange with get_config
• update endpoints to incorporate and use driver ID
• update global endpoints to look up community using community_platform_id and driver_id
• set_config methods for core.utils and httpwrapper.utils (generally called from UI, I think)
• create additional API endpoints (create_api_key, get_communities_for_driver, etc)
• set up authentication for API endpoints
• add user account models, link up to driver accounts
• add tests and update docs as needed
• create UI (probably going to make this a separate PR with its own subtasks)