feat: add demo mode flag

Related to https://kolaente.dev/vikunja/frontend/issues/2453

config.yml.sample

@@ -59,6 +59,9 @@ service:
   # The maximum size clients will be able to request for user avatars.
   # If clients request a size bigger than this, it will be changed on the fly.
   maxavatarsize: 1024
+  # If set to true, the frontend will show a big red warning not to use this instance for real data as it will be cleared out.
+  # You probably don't need to set this value, it was created specifically for usage on [try](https://try.vikunja.io).
+  demomode: false
 
 database:
   # Database type to use. Supported types are mysql, postgres and sqlite.

docs/content/doc/setup/config.md

@@ -333,6 +333,18 @@ Full path: `service.maxavatarsize`
 Environment path: `VIKUNJA_SERVICE_MAXAVATARSIZE`
 
 
+### demomode
+
+If set to true, the frontend will show a big red warning not to use this instance for real data as it will be cleared out.
+You probably don't need to set this value, it was created specifically for usage on [try](https://try.vikunja.io).
+
+Default: `false`
+
+Full path: `service.demomode`
+
+Environment path: `VIKUNJA_SERVICE_DEMOMODE`
+
+
 ---
 
 ## database

pkg/config/config.go

@@ -49,6 +49,7 @@ const (
 	ServiceRootpath        Key = `service.rootpath`
 	ServiceStaticpath      Key = `service.staticpath`
 	ServiceMaxItemsPerPage Key = `service.maxitemsperpage`
+	ServiceDemoMode        Key = `service.demomode`
 	// Deprecated: Use metrics.enabled
 	ServiceEnableMetrics         Key = `service.enablemetrics`
 	ServiceMotd                  Key = `service.motd`
@@ -300,6 +301,7 @@ func InitDefaultConfig() {
 	ServiceEnableEmailReminders.setDefault(true)
 	ServiceEnableUserDeletion.setDefault(true)
 	ServiceMaxAvatarSize.setDefault(1024)
+	ServiceDemoMode.setDefault(false)
 
 	// Auth
 	AuthLocalEnabled.setDefault(true)

pkg/routes/api/v1/info.go

@@ -49,6 +49,7 @@ type vikunjaInfos struct {
 	EmailRemindersEnabled      bool      `json:"email_reminders_enabled"`
 	UserDeletionEnabled        bool      `json:"user_deletion_enabled"`
 	TaskCommentsEnabled        bool      `json:"task_comments_enabled"`
+	DemoModeEnabled            bool      `json:"demo_mode_enabled"`
 }
 
 type authInfo struct {
@@ -92,6 +93,7 @@ func Info(c echo.Context) error {
 		EmailRemindersEnabled:  config.ServiceEnableEmailReminders.GetBool(),
 		UserDeletionEnabled:    config.ServiceEnableUserDeletion.GetBool(),
 		TaskCommentsEnabled:    config.ServiceEnableTaskComments.GetBool(),
+		DemoModeEnabled:        config.ServiceDemoMode.GetBool(),
 		AvailableMigrators: []string{
 			(&vikunja_file.FileMigrator{}).Name(),
 			(&ticktick.Migrator{}).Name(),