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/system.go

@@ -32,6 +32,7 @@ const (
 	SystemWarnMetricLastRunTimestampKey    = "LastWarnMetricRunTimestamp"
 	SystemMetricSupportEmailNotConfigured  = "warn_metric_support_email_not_configured"
 	SystemFirstAdminVisitMarketplace       = "FirstAdminVisitMarketplace"
+	SystemFirstAdminCompleteSetup          = "FirstAdminCompleteSetup"
 	AwsMeteringReportInterval              = 1
 	AwsMeteringDimensionUsageHrs           = "UsageHrs"
 	UserLimitOverageCycleEndDate           = "UserLimitOverageCycleEndDate"