Admin UI: Add ability to clear config values

There are endpoints in the backend that allows you to clear a config value.

They are available at
 * Global admin: `/api/config/:tenantAlias/clear`
 * Tenant admin: `/api/config/clear`

Send a `configFields` parameter which is an array of those elements (`oae-module/feature/element`) which need to be cleared.

It'd be neat if those were to be hooked up in the admin UI