Increase db pool size

We're getting timesout errors from requests trying to get db connections. We
believe that with Falcon using Ruby fibers, it now needs more than one db
connection per thread. Our AWS Aurora instance can handle up-to 5000
connections, and we' currently use less than 100 typically, so we have plenty of
headroom to increase the size.

Jira-Issue: MAV-5188

Author: misaka

config/database.yml

@@ -1,7 +1,7 @@
 default: &default
   adapter: postgresql
   encoding: unicode
-  pool: <%= ENV.fetch("RAILS_MAX_THREADS", 5).to_i %>
+  pool: <%= 5 * ENV.fetch("RAILS_MAX_THREADS", 5).to_i %>
 
 development:
   <<: *default