We'll be using Noticed to handle user notifications

We'll be using Noticed to handle user notifications

This adds the [Noticed](https://github.com/excid3/noticed) gem, generates and
performs its migrations, and adds the base notifier at
`app/notifiers/application_notifier.rb`. Where appropriate, coding style has
been adapted to meet that of this project.

Detail of the migrations:

- Generated with `bin/rails noticed:install:migrations`.
- Merged into a single migration.
- Removed code to detect the type of primary and foreign keys.
- Converted to use Rails 8.1 semantics (originally it was 6.1).