feat: secrets management POC

What would you like to be added: A proof of concept implementation/demo of Secrets Management.

Why is this needed: It will allow us to iterate on the UI/UX whilst the core feature is still in development.

Task list

[ ] Rework the ConfigPage so that it is tabbed (to prepare for "Secrets" tab).
- [x] Create a tabbed layout to replace the current ConfigPage
- [x] Split configPage() usage and app config into two different components
[ ] Create "Secrets" tab (behind featureToggle)
- [ ] Add a basic layout for creating and viewing secrets (view only shows name of secret, no reveal functionality).

grafana / synthetic-monitoring-app