Monolith Part 1: Incorporate id.sc functionality into the app and decommision.

[timcowlishaw]

This also has the nice side effect of sorting out the need for persistent sessions, and all the rails HTML/CSS/JS infrastructure for us for future monolith work.

In the process I am removing the old id.smartcitizen.me Oauth app management interface, as discussed. Session management and the discord callback have been brought into the "monolith", and redirects set up on the id.smartcitizen host.

I've also added tests for the auth functionality.

Merged to staging on 20/10/2024 on the branch staging-2024-10-10.

On deploying, we should clean up oauth apps in the database too, and remember to precompile assets

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ ActionView::Template::Error: The asset "smartcitizen_logo.svg" is not present in the asset pipeline. Ui::SessionsController#new View Issue

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