Simplify the register app form

[michaeljcollinsuk]

Relates to ministryofjustice/analytical-platform#2633

📝 Summary

Post-migration app migration we want to make registering apps self service, therefore the app registration form has been simplified and opened to all logged in users.

This PR:

• Reduces the app registration form to a repo url and datasource.
• Improves validation and error handling when registering an app
• Remove unnecessary JS when looking up a repo, and limiting repos to the ministryofjustice org
• Only an AWS role is created for the app, other settings and Auth0 clients will be created via the manage app page
• Create cloud platform assume role policy when creating app AWS Role

🔍 What should the reviewer concentrate on?

• Changes to the app registration flow

🧑‍💻 How should the reviewer test these changes?

• Run tests with pytest tests
• Run app locally and attempt to register a new app. Example repo that will work: https://github.com/ministryofjustice/data-platform-new-app-reg-test
• Try to register an invalid repo - should receive error message

📚 Documentation status

• Documentation will be added in the future within user guidance - see new app registration epic for related issues

[mitchdawson1982]

LGTM