Use key_value.expirable for the development settings page

Problem/Motivation

The development settings page uses key_value for storing the current state of the toggles. This is good in the sense that it allows a UI without requiring config to change, since usually config gets deployed upwards from development from development to production, while database themselves don't.

However, when taking a site from development to live for the first time, this will usually be done by making an existing environment public, or syncing a database from a staging to production environment.

If development mode is on when the site is moved to production, then it will be on until someone with access to switch it off logs into production and submits the form (or resets it via drush etc.).

I think we could mitigate this by using key_value.expirable instead. If we toggle development mode for some number of days, then even if it makes its way to production, or is toggled on production against the advice on the form to debug something, then even if it's forgotten about, it will eventually get switched off again.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet