Refactor database configs

* Move postgres credentials to ENV vars
* Allow postgres in development
* Allow SQlite in production
* Refactor optional lndhub db config

Co-authored-by: Greg Karékinian <greg@karekinian.com>

.env.example

@@ -1,6 +1,15 @@
 # PRIMARY_DOMAIN=kosmos.org
 # AKKOUNTS_DOMAIN=accounts.example.com
 
+# The default backend is SQLite
+# DB_ADAPTER=postgresql
+# PG_HOST=localhost
+# PG_PORT=5432
+# PG_DATABASE=akkounts
+# PG_DATABASE_QUEUE=akkounts_queue
+# PG_USERNAME=akkounts
+# PG_PASSWORD=
+
 # SMTP_SERVER=smtp.example.com
 # SMTP_PORT=587
 # SMTP_LOGIN=accounts