feat: new configuration system (and schema) (#2819)

config/database.yml

@@ -1,13 +1,13 @@
 default: &default
   adapter: mysql2
   reconnect: true
-  encoding: <%= Postal.config.main_db.encoding %>
-  pool: <%= Postal.config.main_db.pool_size %>
-  username: <%= Postal.config.main_db.username %>
-  password: <%= Postal.config.main_db.password %>
-  host: <%= Postal.config.main_db.host %>
-  port: <%= Postal.config.main_db.port %>
-  database: <%= Postal.config.main_db.database %>
+  encoding: "<%= Postal::Config.main_db.encoding %>"
+  pool: <%= Postal::Config.main_db.pool_size %>
+  username: "<%= Postal::Config.main_db.username %>"
+  password: "<%= Postal::Config.main_db.password %>"
+  host: "<%= Postal::Config.main_db.host %>"
+  port: <%= Postal::Config.main_db.port %>
+  database: "<%= Postal::Config.main_db.database %>"
 
 development:
   <<: *default