Feat/add testing for bookings model

[stringiest]

Resolves #34

Next:

• Add test to make sure that all of your factories are valid
• Add controller tests - check that all pages render and all flash notices are displayed as expected.
• Add a testing doc, setting the testing policy for the app - reference particularly helpful blog posts.

Leave feature tests for now.

Edit: as at 16/11/2024 this also address security vulnerabilities flagged by bundle-audit.