Fix default time zone config

Why:

* Starting the environment, even with just `bundle exec rake`, results
  in the following error:
  ```sh
  Value assigned to config.time_zone not recognized.Run "rake -D time"
  for a list of tasks for finding appropriate time zone names.
  ```

This change addresses the issue by:

* Adding the time zone setting to environment specific configurations,
  defaulting to UTC in development, and using Melbourne in tests.

config/environments/development.rb

@@ -29,6 +29,12 @@ Openfoodnetwork::Application.configure do
   # Expands the lines which load the assets
   config.assets.debug = false
 
+  # Set Time.zone default to the specified zone and make Active Record auto-convert to this zone.
+  # Run "rake -D time" for a list of tasks for finding time zone names. Default is UTC.
+  #
+  # To override this, set the appropriate locale in application.yml
+  config.time_zone = ENV.fetch("TIMEZONE", "UTC")
+
   config.i18n.fallbacks = [:en]
 
   # Show emails using Letter Opener

config/environments/test.rb

@@ -33,6 +33,8 @@ Openfoodnetwork::Application.configure do
   # ActionMailer::Base.deliveries array.
   config.action_mailer.delivery_method = :test
 
+  config.time_zone = ENV.fetch("TIMEZONE", "Melbourne")
+
   # Tests assume English text on the site.
   config.i18n.default_locale = "en"
   config.i18n.available_locales = ['en', 'es']