server: implement data persistence with database

Using databases + sqlite/postgresql depending of what you want.
Use DATABASE_URL to configure

Closes #70

server/reflector/app.py

@@ -2,6 +2,7 @@ from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi_pagination import add_pagination
 from fastapi.routing import APIRoute
+import reflector.db  # noqa
 from reflector.views.rtc_offer import router as rtc_offer_router
 from reflector.views.transcripts import router as transcripts_router
 from reflector.events import subscribers_startup, subscribers_shutdown