Make it easier to add an app

For every app (that uses SSO), there need to be entries in the app and oauthclient_app tables. When adding an app to Stackspin, you currently have to write a database migration (flask alembic) that adds these rows. This is

• not very convenient when adding apps to Stackspin;
• very inconvenient when adding custom apps to a Stackspin cluster, because if you want to add the migration, you have to fork the dashboard code.

We want to make this easier. One way would be to keep the list of apps and the list of oauthclient mappings in a configmap, with an optional second configmap with extensions/overrides for customization. At start-up, the dashboard backend pod would read these configmaps, and add any apps and/or mappings to the database that are still missing.