Heed RAILS_LOG_LEVEL in production config.

This was added to Rails's template for production.rb in
https://github.com/rails/rails/blob/3b83758/railties/lib/rails/generators/rails/app/templates/config/environments/production.rb.tt#L73

We want to be able to control the log level this way so that:

- we don't have to rebuild an app just to change its log level
- we can enable debug logging if we need it to investigate a problem that's
  hard to reproduce
- we can tune the log levels so as not to waste resources logging things that
  we don't care about the rest of the time
- if a change to an app introduces a problem with logspew, we can just
  dial down the log level while someone tracks down the underlying issue

This is a bulk change; all 43 PRs are identical except for apps which had their
loglevel set to `debug`, which we're resetting to `info` as the default.

To approve, use https://github.com/alphagov/bulk-merger (`./review 'Heed
RAILS_LOG_LEVEL in production config.`)

config/environments/production.rb

@@ -35,7 +35,7 @@
 
   # Include generic and useful information about system operation, but avoid logging too much
   # information to avoid inadvertent exposure of personally identifiable information (PII).
-  config.log_level = :debug
+  config.log_level = ENV.fetch("RAILS_LOG_LEVEL", :info)
 
   # Prepend all log lines with the following tags.
   config.log_tags = [:request_id]