Use case onboarding (#19446)

* Add use case onboarding feature flag
* Add endpoints and storage for whether first admin completed setup
* Add migration for FirstAdminSetupComplete

Co-authored-by: Michael Kochell <6913320+mickmister@users.noreply.github.com>
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

model/feature_flags.go

@@ -67,6 +67,9 @@ type FeatureFlags struct {
 
 	NormalizeLdapDNs bool
 
+	// Enable special onboarding flow for first admin
+	UseCaseOnboarding bool
+
 	// Enable Workspace optimization dashboard
 	WorkspaceOptimizationDashboard bool
 
@@ -95,6 +98,7 @@ func (f *FeatureFlags) SetDefaults() {
 	f.InlinePostEditing = false
 	f.BoardsDataRetention = false
 	f.NormalizeLdapDNs = false
+	f.UseCaseOnboarding = false
 	f.WorkspaceOptimizationDashboard = false
 	f.GraphQL = false
 }