Ignore RecordNotFound errors in AppSignal (#1175)

Resolves #1174

# What it does

Prevents AppSignal from treating RecordNotFound errors as reportable
exceptions.

# Why it is important

It's important for us to only alert on real errors or we're more likely
to suffer alert fatigue and miss a true problem.

# Implementation notes

* My first stab at this was reworking `ItemsController#show` to not
raise an exception, but my research suggests that letting an exception
generate the 404 is a normal setup for Rails. We already have
[routes](https://github.com/chicago-tool-library/circulate/blob/ab0a397598cb1d454ea3b495b5d35eab7ad26210/config/routes.rb#L188-L190)
and
[`exception_app`](https://github.com/chicago-tool-library/circulate/blob/main/config/application.rb#L47-L48)
set up to do this.
* I looked into other mechanisms for handling exceptions like
[`rescue_from`](https://edgeapi.rubyonrails.org/classes/ActiveSupport/Rescuable/ClassMethods.html#method-i-rescue_from)
and
[`Rails.error.handle`](https://guides.rubyonrails.org/error_reporting.html),
but I didn't see any obvious advantage to switching everything to either
of these.
* The AppSignal gem can be configured using env vars or a config file.
We currently set only `APPSIGNAL_PUSH_API_KEY` in Heroku, but since
we're expanding our options here it seemed worth switching to file-based
config so we can have comments. This config is based on the one
generated by the `appsignal install` command.

config/appsignal.yml

@@ -0,0 +1,24 @@
+default: &defaults
+  push_api_key: "<%= ENV["APPSIGNAL_PUSH_API_KEY"] %>"
+  name: "Circulate"
+
+  # Errors that should not be recorded by AppSignal
+  # For more information see our docs:
+  # https://docs.appsignal.com/ruby/configuration/ignore-errors.html
+  ignore_errors:
+    # We do error page rendering via Rails config.exceptions_app, which means
+    # that RecordNotFound exceptions coming from controller actions are ok.
+    - ActiveRecord::RecordNotFound
+
+  # See https://docs.appsignal.com/ruby/configuration/options.html for
+  # all configuration options.
+
+# Configuration per environment, leave out an environment or set active
+# to false to not push metrics for that environment.
+development:
+  <<: *defaults
+  active: false
+
+production:
+  <<: *defaults
+  active: true