Regression: Hide appstore splash screen when appstoreenabled=false

[jernst]

Previously implemented in nextcloud/server#13061

The splash screen for the app store is back in 17.0.3 even if disabled, and perhaps earlier.

Steps to reproduce

1. New Nextcloud install
2. Set 'appstoreenabled' => false, in the config.php file, and verify with php occ config:system:get appstoreenabled showing false
3. Log in, through the web, as a new user.
4. Splash screen 2 (I think) talks about the app store, but it should not.