Allow changing the connection pool size

This allows us to tune for UK. The hypothesis from @kristinalim is:

> From what I understand, it can result to Rails processes waiting for
each other to complete, while the DB server can take more simultaneous
connections.

config/database.yml

@@ -1,7 +1,7 @@
 defaults: &defaults
   adapter: postgresql
   encoding: unicode
-  pool: 5
+  pool: <%= ENV.fetch('OFN_DB_POOL', 5) %>
   host: <%= ENV.fetch('OFN_DB_HOST', 'localhost') %>
   username: <%= ENV.fetch('OFN_DB_USERNAME', 'ofn') %>
   password: <%= ENV.fetch('OFN_DB_PASSWORD', 'f00d') %>